Electronic Control System: Other Honda Civic II

INPUT SIGNALS

Computer furnishes input sensors with a reference voltage (5 volts). This voltage signal is then modified by the resistance value of the sensors in each circuit. Some sensors are simple mechanical resistors, while others are temperature sensitive resistors (thermistors). ECU monitors either reference line or return signal to determine status of each sensor. Not all input signals are obtained from sensors. Some input signals are simply monitored circuits which either produce a voltage of their own (O2 and pick-up coils) or already have a voltage signal present.

Atmospheric Pressure Sensor (PA)

The PA sensor converts atmospheric pressure into voltage signals and sends them to ECU.

Battery Voltage Signal

ECU uses the battery voltage signal (alternator FR terminal) to compensate for low voltage conditions which result in injector lag. ECU compensates by increasing injector opening time.

Crank Angle Sensors

On Civic 1.5L and Standard models (TBI), the crank angle sensor consists of a combination TDC/Crank sensor mounted in the distributor housing. The ECU uses input from the crank pick-up coil portion of this sensor to help determine ignition and fuel injector timing for each cylinder. This sensor also furnishes an engine speed (RPM) signal to the ECU. The ECU uses the TDC pick-up coil portion of this sensor to initialize ignition timing upon start-up (cranking) and when crank angle signal is abnormal. (Scheme 48)and (Scheme 49).

In addition to the TDC and Crank pick-up coils inside distributor housing, Civic HF and Si models also utilize a CYL pick-up coil. The CYL portion of this sensor is utilized to detect TDC No. 1 position for triggering of sequential fuel injection. On Civic HF and Si models this sensor is inside distributor along with TDC/CRANK pick up coils. (Scheme 48)and (Scheme 49).

Civic (1.5L & Std.) View of TDC/CRK & CYL Sensors. Scheme 48

Civic CRX HF & Si View of TDC/CRK & CYL Sensors. Scheme 49

Coolant Temperature Sensor (TW)

The TW sensor uses a temperature sensitive resistor (thermistor) to measure differences in coolant temperature. Resistance of thermistor decreases with a rise in coolant temperature.

EGR Lift Sensor

The ECU contains ideal EGR lift specifications for varying operating conditions in its internal memory. EGR lift sensor senses EGR valve position and relays information to ECU. ECU uses this information, along with other sensor input, to determine regulation of EGR control solenoid valve. EGR is not used on Civic 1.5L, standard or Si models.

Ignition Timing Adjusting (ITA) Connector

Used on all models, the ITA is connected to the ECU and TA, TW, PA, throttle position and EGR lift sensors. When jumper is installed across this connector, computer defaults to base timing for adjustment.

Idle Mixture Adjuster (IMA) Sensor

This sensor is used on Civic 1.6L models without catalytic converter and oxygen sensor. The primary function of this sensor is to maintain correct air/fuel ratio at idle. The IMA sensor is mounted on ECU. Turning adjuster changes voltage signal to ECU, which in turn alters fuel discharge duration. Sensor is adjusted and sealed at factory to prevent tampering.

Intake Air Temperature Sensor (TA)

The TA sensor is a thermistor located in the intake manifold. It acts much like the coolant temperature sensor but with a reduced thermal capacity for quicker responses.

Manifold Air Pressure Sensor (MAP)

The MAP sensor converts manifold air pressure readings into electrical voltage signals and sends the signals to ECU.

Oxygen Sensor

The oxygen sensor is mounted in the exhaust manifold where it is exposed to exhaust gases as they exit the combustion chamber. Sensor detects oxygen content of exhaust gas and produces a voltage signal proportional to that content for use by the ECU.

Rich air/fuel ratios will cause oxygen sensor to produce a high signal voltage (up to 1.0 volt), while lean air/fuel ratios will cause sensor to produce a low signal voltage (as low as .1 volt). The ECU receives these input signals and varies fuel injector duration (on time) to maintain proper air/fuel ratios during most operating modes.

Throttle Position Angle Sensor

This sensor is a mechanical variable resistor (potentiometer) connected to throttle shaft. Throttle position sensor varies voltage signal to ECU as throttle angle changes.

Other input signals which may be used on some or all models include signals from A/C clutch, brake switch, clutch switch, cranking signal (fused starter switch), vehicle speed sensor, power steering pressure switch and/or gear position switch(es).

Dashpot Control Vacuum Solenoid (Civic Si)

ECU controls solenoid which regulates vacuum to outer side of idle speed dashpot based upon coolant temperature. When solenoid is energized, vacuum signal to dashpot is blocked. This causes spring pressure to slightly open throttle valve, allowing for cold fast idle. The inner side of idle speed dashpot controls throttle closure speed using spring pressure and an orificed (restricted) manifold vacuum signal. ECU controls outer side of diaphragm only.

Electronic Air Control Valve (EACV)

All models are equipped with an Electronic Air Control Valve. When idle speed is reduced due to electrical or other loads on engine, ECU opens this valve to by-pass additional air into intake manifold. This additional air will allow idle speed to increase to maintain normal idle.

After coolant temperature has reach 104°F (40°C), this valve lowers fast idle speed in steps during engine warm-up. To prevent erratic running after engine first fires, valve is opened during cranking and immediately after starting which by-passes additional air into intake manifold.

Exhaust Gas Recirculation (EGR) Control Solenoid

EGR is used on Civic (HF) models. ECU controls ground circuit to EGR solenoid to regulate flow of recirculated exhaust gases into intake system.

Electric Fuel Pump

The fuel pump is a direct drive type mounted in fuel tank. When ignition is first turned on, ECU supplies a ground circuit for main relay for about 2 seconds. When relay is energized, power is transmitted to fuel pump. When an RPM signal is received by ECU (cranking or running), ECU provides a constant ground circuit for main relay.

Fast Idle Valve

On Civic models, all idle speeds are controlled by ECU, using the EACV.

Fuel Injector

Battery voltage for injector is supplied from main relay and then passes through injector resistor (except Civic 1.5L and standard) before reaching injectors. Injectors are energized when ECU supplies a ground circuit for individual injectors. Because plunger valve lift and fuel pressure are constant, air/fuel ratio is determined solely by length of time that injector is open.

Note. Civic 1.5L and standard models use 2 injectors located in throttle body. All other models have injectors located in intake manifold, one injector for each cylinder.

Ignition Timing

On all models, ignition timing is directly controlled by the ECU based upon input signals from crank angle sensor, throttle angle sensor, coolant sensor and MAP sensor. ECU triggers ignitor, which in turn triggers ignition coil.

Main Relay

One half of main relay is used by ECU to power electric fuel pump. ECU provides a ground for this part of relay when ignition is first turned on (2 seconds), or when an RPM signal is received by ECU (Cranking or running). The second half of the main relay provides power from ignition for the ECU, through a resistor and power for the fuel pump part of relay.

Purge Control Solenoid

Purging evaporative emissions from charcoal canister is controlled by ECU using purge control solenoid. When solenoid is energized, manifold vacuum passes through solenoid to open canister purge control valve diaphragm located on top of canister. Ported vacuum is then used to purge canister.

Tandem Valve Control Solenoid (Civic 1.5L & Std)

The tandem valve is employed on throttle body injected Civic models to improve fuel atomization. This is accomplished by regulating venturi vacuum (to tandem valve actuating diaphragm) through an ECU controlled solenoid valve. This promotes a more complete atomization of fuel from main fuel injector, regardless of airflow rate. Solenoid is energized and venturi vacuum is allowed to activate diaphragm when coolant temperature is greater than 160°F (70°C) or engine speed is greater than 1500 RPM (auto. trans.) or 2000-2800 RPM (man. trans.), depending upon atmospheric pressure.

ENGINE WON'T START

Check spark, fuel supply and EACV.

DIFFICULT COLD START

Check coolant temperature sensor, TDC sensor and all other idle controls.

ROUGH IDLE WHEN WARM

Check EACV, MAP sensor, oxygen sensor, crank angle sensor, throttle position sensor, intake air temperature sensor, atmospheric pressure sensor and EGR and other emission control systems.

IDLE RPM HIGH WHEN WARM

Check EACV and other idle controls. Check MAP sensor, atmospheric pressure sensor, air intake system and emission systems.

IDLE RPM LOW WHEN WARM

Check EACV, air intake system, MAP sensor and other idle controls.

STALLING AFTER WARM-UP

Check EACV, air intake system, MAP sensor, crank angle sensor, coolant temperature sensor, throttle position sensor, other idle controls, fuel supply and EGR control system.

STALLING DURING WARM-UP

Check EGR control system, EACV, MAP sensor, throttle position sensor, crank angle sensor, vehicle speed sensor, air intake and fuel supply.

LACK OF POWER

Check for dragging/binding brakes. Check fuel supply, MAP sensor, crank angle sensor, coolant temperature sensor, atmospheric pressure sensor, vehicle speed sensor, air intake system and control system.

No PGM-FI Warning Light

1. Turn ignition on. If oil pressure light is on, go to step 2). If light is off, inspect fuse No. 1. If fuse is okay, repair open circuit in Yellow wire between fuse and combination meter.
2. Turn ignition off. Connect system checker harness between ECU and connector. Connect terminal B6 to ground. Turn ignition on. If warning light is now on, go to step 3). If light is off, replace warning light bulb or repair open in wire between ECU terminal B6 and combination meter.
3. Individually, connect terminal A2 and then terminal A4 to terminal B6. If warning light fails to illuminate, repair open in Black A2 or A4 wires between ECU and ground.
4. If warning light illuminated in step 3), substitute a known good ECU. If condition is rectified, replace original ECU.

Check Engine Light On, LED Does Not Flash

1. Connect system checker harness between ECU and ECU connectors. Disconnect "B" connector from ECU only, not from main wire harness. Turn ignition on. If Check Engine light remains on, repair short to ground in Green/Orange wire between ECU terminal B6 and combination meter.
2. If Check Engine light goes out with ECU "B" connector unplugged, reconnect "B" connector. Individually, jumper terminal A2 to terminal A16 and then to terminal A18. If Check Engine light illuminates, go to step 3). If Check Engine light does not illuminate, repair open in Black/Red wire between ECU terminal A18 and ground or repair open in Brown/Black wire between ECU terminal A16 and ground.
3. Measure voltage between terminal A18 (negative) and terminals A13 (positive) and A15 (positive). If battery voltage is present, go to step 4). If battery voltage is not present, repair open in Yellow/Black wires between ECU terminals A13 and A15 and main relay. Check main relay and wiring connectors at main relay.
4. Turn ignition off. Disconnect 3-wire connector at MAP sensor. Start engine. If LED does not indicate code for the disconnected sensor, replace sensor. Reconnect 3-wire connector. Repeat procedure with, throttle angle sensor, PA sensor, and finally idle mixture adjuster sensor (without catalyst). If all sensor codes are displayed, turn ignition off. With system checker harness installed between ECU and ECU connector, disconnect "C" connector from ECU only, not from main harness.
5. Check for continuity between ground and terminal C13. Check for continuity between ground and terminal C15. If continuity exists, repair short to ground in Yellow/Red wire between ECU terminal C15 and MAP sensor, repair short to ground in Yellow/White wire between ECU terminal C13 and altitude sensor or throttle angle sensor, or repair short to ground in Yellow/White wire between ECU terminal C13 and idle mixture adjuster sensor (without catalyst).
6. If continuity did not exist in step 5), substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 1 Or 2: O2 SENSOR

1. Turn off ignition. Reset ECU by removing hazard fuse in main fuse box for 10 seconds. Start engine. Check fuel pressure at fuel filter. See appropriate FUEL INJECTION article in FUEL SYSTEMS section. Repair fuel system if not within specifications. If fuel pressure is within specifications, proceed to next step.
2. Bring engine to operating temperature (cooling fan comes on). Hold engine speed at 1500 RPM (minimum) for 15 minutes, without closing throttle completely during this time. Check if PGM-FI or Check Engine Warning Light is on. If not on, problem is intermittent. Test drive and check light again.
3. If light is on, disconnect indicated oxygen sensor connector. Attach voltmeter to vehicle ground and oxygen sensor connector. With engine at normal operating temperature, momentarily open throttle wide open, then close throttle.
4. Voltage at wide open throttle should be greater than .6 volt, and less than .4 volt at closed throttle. If not within specifications, replace oxygen sensor. If voltage is within specifications, proceed to next step.
5. Turn engine off and attach system checker harness between ECU and ECU connector. Using a voltmeter, connect negative lead to connector terminal A18.
6. On Civic models, attach positive voltmeter lead to connector terminal C16.
7. Start engine and warm to operating temperature. Momentarily open throttle wide open, then close throttle. Voltage at wide open throttle should be greater than .6 volt, and less than .4 volt at closed throttle. If not within specifications, repair wire between ECU and oxygen sensor. If within specifications, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 3: MAP SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 3-wire MAP sensor connector. Turn ignition on. Measure voltage between Yellow/Red (pos.) and Green/White (neg.) terminals. Go to step 4) if reading is about 5 volts. If not, measure voltage between Yellow-Red (pos.) terminal and ground. If voltage is still not 5 volts, go to step 3). If voltage is about 5 volts, repair open in Green/White wire between ECU terminal C14 and MAP sensor.
3. Turn ignition off. Connect system checker harness between ECU and ECU connector. Measure voltage between terminal C15 (pos.) and C14 (neg.) with ignition on. If voltage reading is about 5 volts, repair open Yellow/Red wire between ECU terminal C15 and MAP sensor. If voltage is not about 5 volts, substitute a known good ECU. If condition is rectified, replace original ECU.
4. Measure voltage between White (pos.) and Green/White (neg.) terminals. If reading is not about 5 volts, go to step 6). If measured voltage is about 5 volts, turn ignition off. Connect system checker harness between ECU and ECU connector. Reconnect 3-wire MAP connector. Turn ignition on. Measure voltage between terminal C11 (pos.) and C14 (neg.). If there is not about 3 volts present, replace MAP sensor.
5. If there is about 5 volts present between C11 and C14, substitute a known good ECU. If condition is rectified, replace original ECU.
6. If voltage measured between White (pos.) and Green/White (neg.) terminals in step 4) was not about 5 volts, turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminal C11 (pos.) and C14 (neg.). If reading is not about 5 volts, repair short in White wire between ECU terminal C11 and MAP sensor. If voltage is about 5 volts, repair open in White wire between ECU terminal C11 and MAP sensor.

CODE 4: TDC/CRANK SENSOR (CIVIC 1.5L & STANDARD)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 6-wire TDC/Crank sensor connector at distributor. Measure resistance between terminals "D" and "E". (Scheme 50) If resistance is within specification, check for continuity between ground and terminals "D" and "E". If continuity exists, sensor is shorted to ground. Replace distributor assembly.
3. If continuity to ground did not exist in step 2), reconnect 6-wire connector. Connect system checker harness between ECU and ECU connector. Measure resistance between terminals B10 and B12 (DOHC) or terminals C1 and C2 (1.5L). If resistance is not 350-550 ohms, repair Orange and/or White wires.
4. If resistance is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 4: TDC/CRANK/CYL SENSOR (CIVIC HF & Si)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Unplug 8-wire TDC/CRANK/CYL sensor connector at distributor. Measure resistance between terminals "C" and "D". If continuity exists, sensor is shorted to ground. Replace distributor assembly.
3. If continuity to ground did not exist in step 2), reconnect 8-wire connector. Connect system checker harness to main harness only, not to ECU. Measure resistance between terminals B10 and B12. If resistance is not 350-550 ohms, repair Orange and/or White wires.
4. If resistance is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.

Civic HF & Si TDC/CRANK/CYL Sensor Connector. Scheme 50

CODE 5: MAP SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Connect hand-held vacuum pump/gauge to MAP hose at throttle body. Apply vacuum. If vacuum holds, go to step 3). If vacuum does not hold, attach vacuum pump/gauge directly to MAP sensor. Apply vacuum. If vacuum does not hold, replace MAP sensor. If vacuum holds, check MAP sensor vacuum hose for cracks, splits or looseness. Repair as necessary.
3. Check vacuum supply to MAP sensor with engine running. If vacuum is not present, repair hose or remove restriction from throttle body. If vacuum supply is okay, turn ignition off. Connect system checker harness between ECU and ECU connector. Measure voltage between terminals C11 (pos.) and C14 (neg.). If voltage reading is about 3 volts, go to next step. If voltage is not about 3 volts, check for open in White wire between MAP sensor and ECU. If wire is okay, replace MAP sensor.
4. Substitute a known good ECU. If condition is rectified, replace original ECU.

Scheme 51

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. IF light is on, warm engine to normal operating temperature (cooling fan on). Disconnect C101 (1.5L and standard) or C151 (HF and Si) connector from C210 connector. (Scheme 51) Measure resistance between Red/White wire terminal and Green/White wire terminal of C101 or C151 connector, on sensor side of harness. If resistance value is not 200-400 ohms, inspect for short or open in Red/White or Green/White wire between connector and coolant sensor. If wires are okay, replace coolant sensor.
3. If resistance values are within specification, reconnect connectors. Disconnect 2-wire connector directly from coolant sensor. Measure voltage between ground and Red/White wire on sensor harness. If voltage is not about 5 volts, go to step 5). If voltage reads about 5 volts, measure voltage between Red/White (pos.) and Green/white (neg.) wires. If about 5 volts is not present, repair open in Green/-White wire between ECU and coolant sensor.
4. If voltage reading from Green/White wire to Red/White wire of coolant sensor harness was about 5 volts, substitute a known good ECU. If about 5 volts is now read, replace original ECU. (Scheme 51): Connector Identification
5. If voltage reading from ground to Red/White wire of coolant sensor harness was not about 5 volts in step 3), turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C6 (pos.) and C12 (neg.). If about 5 volts is present, repair open in Red/White wire between ECU terminal C6 and coolant sensor.
6. If voltage reading is not about 5 volts, disconnect "C" connector from main wiring harness only, not at ECU. Measure voltage between terminals C6 (pos.) and C12 (neg.). If about 5 volts is now indicated, repair short in Red/White wire between ECU terminal C6 and sensor.
7. If about 5 volts is not indicated, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 7: THROTTLE ANGLE SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 3-wire connector at throttle position sensor. Turn ignition on. Measure voltage between Yellow/White (pos.) and Green/White (neg.) terminals. If voltage is not about 5 volts, go to step 5). If voltage is about 5 volts, turn ignition off. Reconnect sensor connector. Connect system checker harness between ECU and ECU connector.
3. Turn ignition on. Measure voltage between terminals C7 (pos.) and C12 (neg.). Voltage should read .5 volt at closed throttle, and about 4.5 volts at wide open throttle, with a smooth transition between the lower and upper voltage specification as the throttle is depressed.
4. If sensor does not respond as indicated in step 3), replace throttle sensor or repair open or short in Red/Blue wire between ECU terminal C7 and throttle sensor. If throttle sensor responds as indicated, substitute a known good ECU. If condition is rectified, replace original ECU.
5. If voltage reading between Yellow/White (pos.) and Green/White (neg.) terminals in step 2) was not about 5 volts, measure voltage between ground and Yellow/White wire terminal. If voltage is now about 5 volts, repair open in Green/White wire between ECU terminal C12 and throttle sensor. If about 5 volts is not present between ground and Yellow/White wire, turn ignition off.
6. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C13 (pos.) and C12 (neg.). If voltage is still not about 5 volts, go to next step. If voltage is about 5 volts, repair open Yellow/White wire between ECU terminal C13 and throttle sensor.
7. Substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 8: TDC/CRANK SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect sensor connector at distributor. Measure resistance between terminals "B" and "C" (standard and DOHC) or terminals "A" and "B" (HL and Si). (Scheme 48), (Scheme 49) and (Scheme 50). If resistance is not 350-550 ohms, replace distributor assembly. If resistance value is within specification, individually check for continuity between ground and the same respective terminals. If continuity exists, sensor is shorted to ground. Replace distributor assembly.
3. Reconnect sensor connector. Connect system checker harness to main wiring harness only, not to ECU. Measure resistance between terminals C3 and C4. If resistance is not 350-550 ohms, repair open or short in Orange/Blue and/or White/Blue wire. If resistance value is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 9: TDC/CRANK/CYL SENSOR (CIVIC HF & Si)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 8-wire sensor connector nearest the sensor. Measure resistance between Blue/Green and Blue/Yellow wire of sensor. If reading is not 350-550 ohms, replace distributor assembly. If reading is within specification, proceed to next step.
3. Check continuity between ground and Blue/Green and Blue/Yellow wire sensor terminals. If continuity exists, replace distributor assembly. If no continuity exists, proceed to next step.
4. Reconnect sensor connector. Attach system checker harness to main wire harness only, not to ECU. Check resistance between connector terminals C1 and C2. Resistance should be 350-550 ohms.
5. If resistance is not within specification, repair open in Blue/Green and/or Blue/Yellow wires. If resistance is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 9: CYL SENSOR (CIVIC STANDARD)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 2-wire sensor connector. Measure resistance across sensor terminals. If reading is not 700-1000 ohms, replace CYL sensor assembly. If reading is within specification, proceed to next step.
3. Check continuity between ground and each sensor terminal. If continuity exists, replace CYL sensor assembly. If no continuity exists, proceed to next step.
4. Reconnect sensor connector. Attach system checker harness to main wire harness only, not to ECU. Check resistance between connector terminals C1 and C2. Resistance should be 700-1000 ohms.
5. If resistance is not within specification, repair open in Blue/Green and/or Blue/Yellow wires. If resistance is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 10: AIR TEMPERATURE (TA) SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Measure resistance across TA sensor terminals. If reading is not 1000-4000 ohms, replace sensor. If resistance is within specification, measure voltage between ground and Red/Yellow wire terminal. If reading is about 5 volts, go to step 3). If reading is not about 5 volts, go to step 4).
3. Measure voltage between same wire (pos.) and Green/White (neg.) wire terminal. If voltage is not about 5 volts, repair open in Green/White between ECU terminal C12 and sensor. If reading is about 5 volts, substitute a known good ECU. If condition is rectified, replace original ECU.
4. If voltage in step 2) was not about 5 volts, turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C5 (pos.) and C12 (neg.). If voltage reading is about 5 volts, repair open in Red/Yellow wire between ECU terminal C5 and sensor.
5. If reading was not about 5 volts, disconnect "C" connector from the main wire harness only, not the ECU. Measure voltage between terminals C5 (pos.) and C12 (neg.). If voltage reading is about 5 volts, repair short in Red/Yellow wire between ECU terminal C5 and sensor.
6. If voltage is not about 5 volts, substitute a known good ECU. If voltage is now correct, replace original ECU.

CODE 11: IDLE MIXTURE ADJUSTER (IMA) (CIVIC WITHOUT CATALYST)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 3-wire connector at sensor. Measure resistance across Yellow/White and Green/White wires of sensor. If resistance is not 400-6000 ohms, replace sensor. Measure resistance across Yellow/White and Brown wires of sensor. If resistance is not 400-6000 ohms, replace sensor.
3. If resistance values are within specification, turn ignition on. Measure voltage between Yellow/White (pos.) and Green/White (neg.) terminals of sensor harness. If voltage is not about 5 volts, go to step 5). If voltage is about 5 volts, turn ignition off. Connect system checker harness between ECU and ECU connector.
4. Turn ignition on. Measure voltage between terminals B20 (pos.) and C12 (neg.). If voltage reading is not .5-4.5 volts, repair open or short in Brown wire between ECU terminal B20 and IMA sensor. If voltage is within specification, substitute a known good ECU. If condition is rectified, replace original ECU.
5. Measure voltage between ground and Yellow/White wire of sensor harness. If voltage is now about 5 volts, repair open in Green/White wire between ECU terminal C12 and IMA sensor. If voltage is not about 5 volts, turn ignition off.
6. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C13 (pos.) and C12 (neg.). If voltage is now about 5 volts, repair open in Yellow/White wire between ECU terminal C13 and IMA sensor.
7. If voltage is not about 5 volts, substitute a known good ECU. If voltage reading is now correct, replace original ECU.

Scheme 52

1. Check harness and connections between EGR valve and ECU. Verify vacuum supply to EGR control solenoid. Check vacuum hoses for cracks, splits or looseness. Repair as necessary. Warm engine to normal operating temperature (cooling fan on). Disconnect vacuum hose from EGR valve. If vacuum does not hold and engine does not stall, replace faulty EGR valve or clean blocked EGR passages.
2. If vacuum held and engine stalled in step 1), remove control box cover. Disconnect vacuum hose between air chamber and solenoid valve. Connect vacuum gauge to air chamber. If there is not about 8 in. Hg, replace CVC. If vacuum is present, reconnect hose between vacuum chamber and EGR solenoid.
3. If vacuum is now present at EGR valve, check for short in Red wire between ECU terminal A10 and EGR control solenoid. If wire is okay, replace solenoid. If vacuum is not present, disconnect 4-wire connector from control box. (Scheme 52) Jumper battery voltage to terminal "D" and ground to terminal "C" at control box (solenoid side of harness). Check EGR valve for 8 in. Hg. If vacuum is present, go to step 5).
4. Connect and disconnect ground from terminal "C" of solenoid harness. If EGR solenoid does not click, replace solenoid. If solenoid does click, repair blockage in vacuum port of solenoid.
5. Measure voltage between ground and Black/Yellow wire terminal of harness. If battery voltage is not present, repair open in Black/Yellow wire between solenoid valve and No. 14 fuse. If battery voltage is present, turn ignition off. Connect test harness between EGR lift sensor and sensor connector.
6. Turn ignition on. Measure voltage between Red (pos.) and Green (neg.) terminals of harness. If about 5 volts is indicated, go to step 8). If 5 volts is not indicated, measure voltage between ground and Red (pos.) harness terminal. If 5 volts is indicated, repair open in Green/White wire between ECU terminal C12 and sensor. (Scheme 52): Civic (HF) EGR Solenoid Terminal Identification
7. If 5 volts is not indicated, turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C13 (pos.) and C12 (neg.). If voltage is about 5 volts, repair open in White/Yellow wire between ECU terminal C13 and sensor. If 5 volts is not indicated, substitute a known good ECU. If condition is rectified, replace original ECU.
8. Measure voltage between White (pos.) and Green (neg.) terminals of harness. If about one volt is not indicated, replace EGR valve or repair short in Yellow wire between ECU terminal C8 and EGR valve lift sensor. If about one volt is indicated, turn ignition off.
9. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C8 (pos.) and C12 (neg.). If about one volt is not indicated, repair open Yellow wire between ECU terminal C8 and sensor.
10. Disconnect "A" connector of system checker harness from ECU only, not main harness. Measure voltage between terminals A10 (pos.) and A2 (neg.). If battery voltage is not present, repair open Red wire between ECU terminal A10 and EGR control solenoid.
11. If battery voltage is indicated, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 13: ATMOSPHERIC (PA) SENSOR

1. Turn off ignition. Remove hazard fuse in main fuse box to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C13 (pos.) and C12 (neg.). If about 5 volts is red, go to step 3). If not, substitute a known good ECU. If correct voltage is now indicated, replace original ECU.
3. Measure voltage between terminals C9 (pos.) and C12 (neg.). If about 3 volts is not present, go to step 4). If about 3 volts is present, substitute a known good ECU. If condition is rectified, replace original ECU.
4. Disconnect connector from atmospheric pressure sensor. Measure voltage between terminals C9 (pos.) and C12 (neg.) on harness side. If about 5 volts is indicated, go to step 5). If about 5 volts is not indicated, repair short in Red/White wire between ECU terminal C9 and sensor.
5. Measure voltage between Yellow/White and Green/White (neg.) wires. If reading is not about 5 volts, go to step 6). If about 5 volts is present, measure voltage between Red/White and Green/White (neg.) terminals. If about 5 volts is not read, repair open Red or Red/White wire between ECU terminal C9 and sensor. If about 5 volts is red, replace sensor.
6. Measure voltage between ground and Yellow/White wire of harness. If about 5 volts is not present, repair open in White/Yellow or Yellow/White wire between ECU terminal C13 and sensor. If about 5 volts is present, repair open Green/White wire between ECU terminal C12 and sensor.

CODE 14: IDLE CONTROL SYSTEM EACV

1. Turn off ignition. Remove hazard fuse in main fuse box to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect Electronic Air Control Valve (EACV) connector. Check resistance between the 2 connector terminals on valve.
3. Resistance should be 8-15 ohms. If not within specification, replace EACV. If resistance is within specification, proceed to next step.
4. Check for continuity to ground on each EACV connector terminal. If continuity exists, replace EACV. If no continuity exists, turn ignition on. Connect a voltmeter between Black/Yellow (pos.) and Blue/Yellow (neg.) terminals.
5. Check for battery voltage. If no battery voltage exists, proceed to step 6). If battery voltage exists, disconnect connector from ECU and again check for battery voltage. If battery voltage still does not exist, substitute a known good ECU. If condition is rectified, replace original ECU. If battery voltage does exist, repair short in Blue/Yellow wire between ECU terminal A11 and EACV.
6. If no battery voltage existed in step 5), measure voltage between ground and Black/Yellow wire terminal. If battery voltage now exists, proceed to step 7). If battery voltage still does not exists, repair open Black/Yellow wire between fuse No. 14 and EACV.
7. If battery voltage exists, turn ignition off. Reconnect EACV connector. Connect system checker harness "A" connector to main wire harness only, not ECU. Turn ignition on. Using a voltmeter, measure voltage between connector terminals A11 (pos.) and A2 (neg.). If voltage does not exist, repair open in wire between ECU terminal A11 and EACV.
8. If battery voltage exists, connect and disconnect terminal A11 to A2. If EACV does not click when connector is connected and disconnected, replace EACV. If EAVC clicks, substitute a known good ECU. If condition is rectified, replace original ECU.

CODE 15: IGNITION OUTPUT SIGNAL

1. Turn off ignition. Remove hazard fuse in main fuse box to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Disconnect 2-wire connector at distributor connector at ignitor. Turn ignition on. Measure voltage between ground and Black/Yellow wire terminal on ignition side of harness. If battery voltage is not present, repair open Black/Yellow wire between connector and ignition switch.
3. If battery voltage is present, turn ignition off. Reconnect 2-wire or 6-wire connector. Connect system checker harness between ECU and ECU connector. Turn ignition on. Individually, measure voltage between terminal A18 (neg.) and terminals B15 and B17. If battery voltage is present, go to step 4). If battery voltage is not present, repair open in wires between distributor and ECU terminals B15 or B17 or replace faulty ignitor unit.
4. Substitute a known good ECU. If condition is rectified, replace original ECU.
5. Turn ignition off. Disconnect 6-wire ignitor connector and connectors from ECU only, not main harness. Check for continuity between White wires and ECU terminals B15 and B17 and ignitor. If continuity does not exist, repair as necessary. If continuity does exist, check for continuity between ground and White wires of 6-wire connector. If continuity exists, repair short to ground. If continuity does not exist, replace ignitor.

CODE 16: FUEL INJECTOR CIRCUIT (CIVIC HF & Si)

1. Turn off ignition. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Attempt to start engine. If engine does not start, go to step 3). Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Test drive and check again. If light is on, using a stethoscope, check for clicking sound at each injector while engine is idling. If all injectors click, substitute a known good ECU. If condition is rectified, replace original ECU.
3. Turn ignition off. If engine does not start, check resistance of all injectors. If engine does start, check resistance of all injectors. If engine does start, check resistance of injectors which did not click. If resistance value is not 1.5-2.5 ohms, replace injector(s). If resistance values are within specification, turn ignition on. Measure voltage between ground and Red/Black wire at injector.
4. IF battery voltage is not present, go to step 7). If battery voltage is present, check voltage at the following terminals: Injector No. 1 - Red/Black (pos.) and Brown (neg.). Injector No. 2 - Red/Black (pos.) and Red (neg.). Injector No. 3 - Red/Black (pos.) and Light Blue (neg.). Injector No. 4 - Red/Black (pos.) and Yellow (neg.). If battery voltage is not present, go to step 5). If battery voltage is present, disconnect ECU connector. If battery voltage is still present, repair short in wires A1 (injector 1), A3 (injector 2), A5 (injector 3) or A7 (injector 4) between ECU and injector. If battery voltage is not present, substitute a known good ECU. If battery voltage is now present, replace original ECU.
5. Reconnect injector connector(s). Connect system checker harness between ECU and ECU connector. Measure voltage between negative terminal A2 and the following positive terminals: Injector No. 1 - A1 Injector No. 2 - A3 Injector No. 3 - A5 Injector No. 4 - A7
6. If battery voltage is not present, repair open in wire A1 (injector 1), A3 (injector 2), A5 (injector 3) or A7 (injector 4) between ECU and injector. If battery voltage is present, substitute a known good ECU. If condition is rectified, replace original ECU.
7. Turn ignition off. Disconnect injector resistor connector. Turn ignition on. Measure voltage between ground and Yellow/Black wire terminal. If battery voltage is present, replace injector resistor. If battery voltage is not present, repair open in Yellow/Black wire between injector resistor and main relay.

Engine Starts And Runs

1. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Start engine and hold engine speed at 2000 RPM for one minute. Verify if "CHECK ENGINE" light is on and if trouble code(s) is present. If not, problem is intermittent. Drive vehicle and retest.
2. If "CHECK ENGINE" light is on and trouble code 16 is present, turn ignition off. Disconnect 2-wire connector from main injector. Measure resistance between injector terminals. If resistance is not within 0.6-1.6 ohms, replace injector. If resistance is as specified, connect voltmeter positive lead to main injector's Yellow/Black wire and voltmeter negative lead to ground.
3. Turn ignition on. If battery voltage is not present for 2 seconds, repair open in Yellow/Black wire between main injector and main relay. If battery voltage is present for 2 seconds, turn ignition off. Disconnect 2-wire connector from auxiliary injector.
4. Measure resistance between auxiliary injector terminals. If resistance is not 6-10 ohms, replace auxiliary injector. If resistance is as specified, connect voltmeter positive lead to Yellow/Black wire of auxiliary injector and voltmeter negative lead to ground.
5. Turn ignition on. Battery voltage should be present for 2 seconds. If not, repair open Yellow/Black wire between auxiliary injector and main relay. If battery voltage is present for 2 seconds, go to step 6).
6. Connect voltmeter positive lead to Yellow wire of auxiliary injector connector and negative voltmeter lead to ground. Reading should be approximately 10 volts. If okay, go to step 7). If not, repair open in Yellow wire between auxiliary injector and ECU terminal A3. If wire is okay, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU.
7. Measure voltage between main injector Red wire and ground. If approximately 10 volts is present, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU.
8. If from step 7), voltage is not as specified, repair open in Red wire between main injector and ECU terminals A5 and A7. If wire is okay, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU.

Engine Will Not Start

1. Remove hazard fuse in main fuse box for 10 seconds to reset ECU. Crank engine for at least 15 seconds to reproduce trouble code in ECU memory. Verify if "CHECK ENGINE" light is on and if trouble code(s) is present. If not, perform SPARK test in BASIC TESTING article.
2. If "CHECK ENGINE" light is on and trouble Code 16 is present, turn ignition off. Disconnect 2-wire connector from auxiliary injector. Check resistance between the injector terminals. Resistance should be 6-10 ohms. If not, replace auxiliary injector.
3. If resistance was okay, turn ignition switch on. Measure voltage between auxiliary injector connector Yellow wire and ground. If about 10 volts is present, go to step 4). If not, repair open or short in Yellow wire between auxiliary injector and ECU terminals A1 and A3. If wire is okay, substitute a known good ECU and retest. If specified voltage is now available, replace original ECU.
4. Disconnect 2-wire connector from main injector. Measure voltage between main injector's Red wire and ground. If about 10 volts is present, go to step 5). If not, repair open or short in Red wire between main injector and ECU terminals A5 and A7. If wire is okay, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU.
5. Turn ignition off. Connect voltmeter positive lead to Yellow/Black wire of auxiliary injector and voltmeter negative lead to ground. Turn ignition on. If battery voltage is present for 2 seconds, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU. If battery voltage is not present for 2 seconds, go to step 6).
6. Turn ignition off. Connect ECU test harness between ECU and ECU connector. (Scheme 48) Jumper ECU terminals A18, A12 and A14 to each other. Turn ignition on. Check for battery voltage at Yellow/Black wire of auxiliary injector connector.
7. If battery voltage is not present, check and repair open in Black/Yellow wire between auxiliary injector and main relay or Green/Black wire between main relay and ECU terminals A12 and A14. If wires are okay, replace main relay.
8. If battery voltage was present from step 6), repair Black wire between ground on thermostat housing and ECU terminals A2 and A4. If wire is okay, substitute a known good ECU and retest. If specified voltage is now present, replace original ECU.

CODE 17: VEHICLE SPEED SENSOR

1. Block drive wheels and set parking brake. Raise front of vehicle and support with safety stands. With ignition off, connect system checker harness between ECU and ECU connector. Turn ignition on.
2. Slowly rotate left front wheel while measuring voltage between terminals B16 (pos.) and A18 (neg.). If voltmeter pulses between zero and 5 volts, substitute a known good ECU. If condition is rectified, replace original ECU.
3. If voltmeter does not pulse between zero and 5 volts, repair open or short in Yellow/Red wire between ECU terminal B16 and speed sensor or replace faulty speed sensor.

CODE 19: LOCK-UP CONTROL SOLENOID

1. Turn ignition off and remove hazard fuse in main fuse box for 10 seconds to reset ECU. Drive vehicle and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Connect system checker harness between ECU and ECU connector. Disconnect "A" connector from ECU only, not from main harness. Turn ignition on. Measure voltage between terminals A17 (pos.) and A18 (neg.). If battery voltage is present, go to step 4).
3. If battery voltage is not present, inspect No. 14 fuse. If fuse is okay, repair open in Black/Yellow wire between ECU terminal A17 and No. 14 fuse.
4. Turn ignition off. Disconnect connector from lock-up control solenoid valve. Check terminal A8 at ECU harness for continuity to ground. If continuity exists, repair short in wire between ECU and solenoid valve.
5. If continuity does not exist, reconnect solenoid connector. Measure resistance between terminal A8 and A18. If resistance is 14-25 ohms, substitute a known good ECU. If condition is rectified, replace original ECU.
6. If resistance is not to specification, unplug solenoid connector. Check for continuity in Yellow wire between ECU and solenoid connector. If continuity does not exist, repair open in Yellow wire. If continuity does exist, replace faulty lock-up solenoid valve.

CODE 20: ELECTRICAL LOAD DETECTOR

1. Turn ignition off and remove hazard fuse in main fuse panel for 10 seconds to reset ECU. Start engine and observe Check Engine light. If light is off, problem is intermittent. Test drive and check again.
2. If light is on, turn ignition off. Open lid of main fuse panel in engine compartment. Disconnect 3-wire electrical load detector. Turn ignition on. Measure voltage between Black/Yellow (pos.) and Black (neg.) wire terminals of harness. If battery voltage is present, go to step 4).
3. If battery voltage is not present, measure voltage between Black/Yellow wire and ground. If battery voltage is now present, repair open in Black wire between connector and ground. If battery voltage is not present, repair open Black/Yellow wire between No. 14 fuse and connector.
4. Turn ignition off. Connect system checker harness between ECU and ECU connector. Disconnect "B" connector from ECU only, not main harness. Connect Green/Red terminal of load detector harness connector to ground. Check for continuity between ECU terminal B19 and ground. If continuity does not exist, repair open Green/Red wire between ECU connector terminal B19 and load detector connector.
5. Reconnect "B" connector to ECU. Turn ignition on. Measure voltage between Green/Red (pos.) and Black (neg.) terminals of load detector harness. If 4.5-5 volts is present, go to step 7). If 4.5-5 volts is not present, disconnect "B" connector from main wiring harness, not ECU. Measure voltage between terminals B19 (pos.) and A18 (neg.).
6. If voltage is now 4.5-5 volts, repair short in Green/Red wire between ECU terminal B19 and load detector connector. If voltage is not 4.5-5 volts, substitute a known good ECU. If correct voltage is now available, replace original ECU.
7. Reconnect load detector connector. Measure voltage between terminals B19 (pos.) and A18 (neg.) under the following conditions. Headlight switch in first position - Voltage should be 2.5 -3.5 volts. Headlight switch in second position - Voltage should be 1.5 -2.5 volts.
8. IF voltage is not as noted replace faulty electrical load detector. If voltage responds as indicated, substitute a known good ECU. If condition is rectified, replace original ECU.

Civic CRX (Standard) Wiring Schematic. Scheme 53

Civic 2WD Wiring Schematic. Scheme 54

Civic CRX (HF & Si) Wiring Schematic. Scheme 55

Civic 4WD Wiring Schematic. Scheme 56