Engine Controls - Tests W/codes: Other Geo Prizm II

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

HARD FAILURES

Hard failures cause MIL to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause must be found using diagnostic code charts. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but loss of good driveability will most likely occur.

INTERMITTENT FAILURES

Intermittent failures cause MIL to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, it will be erased from ECM memory. Intermittent failures may be caused by faulty sensor, connector or wiring. See INTERMITTENTS in TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

RETRIEVING CODES (NON-SCAN)

All vehicles are equipped with a Malfunction Indicator Light (MIL), located on instrument cluster. Light comes on, as a bulb check, when ignition switch is turned to ON position and engine is not running. Light should not flash at this time and should go out when engine is started.

If MIL remains on or flashes with engine running, self-diagnostic system has detected a problem. If problem goes away, light will go out after 10 seconds, but a fault code will be stored in ECM memory.

All trouble codes except Code 16 (PCM control signal on Prizm LSi), Code 43 (crank signal) and Code 51 (switch condition signal) are stored in ECM memory. Codes 16, 43 and 51 are erased from memory as soon as ignition is turned off. To detect an active Code 16, 43 or 51, turn ignition switch to ON position (engine off) with diagnostic request terminal TE1 in Data Link Connector (DLC) grounded. see scheme 1 DLC is located on left shock tower. If ignition switch is turned to OFF position at any time during this procedure, a hard or active Code 16, 43 or 51 will be lost.

CLEARING CODES

Turn ignition switch to OFF position. Disconnect negative battery cable, or remove EFI F-HTR fuse in underhood fuse/relay block, for at least 30 seconds. ECM fault code memory is now clear. Connect negative battery cable or install EFI F-HTR fuse. If negative battery terminal is removed to clear ECM memory, clock and radio memory will also be erased.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. If vehicle does not pass on-board diagnostic system check, information received by scan tester may be invalid.

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See TYPICAL DATA VALUES table.

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the ECM. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See TYPICAL DATA VALUES table.

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

SCAN TESTER DATA

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

Note. On Prizm and Prizm LSi, typical data values information is not available from manufacturer.

CHART A1-1 (PRIZM) - NO MALFUNCTION INDICATOR LIGHT

Malfunction Indicator Light (MIL) should remain on when ignition switch is in ON position and engine is not running. Battery voltage is applied to indicator bulb. ECM controls indicator bulb and turns it on by providing a ground path through Red/Yellow wire to ECM.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks if engine will start.
2. Checks if ECM is receiving power.
3. Checks for open in Red/Yellow wire to ECM or a faulty ECM.
4. Checks if EFI F-HTR relay switch is receiving power.
5. Checks if ECM has a good ground.
6. Checks for open in Red/Yellow wire to instrument cluster.
7. Checks if MIL is receiving power.
8. Checks if EFI F-HTR relay coil is receiving power.
9. Checks for open in EFI F-HTR ground wire.
10. Checks for open in Black/Orange wire between junction block No. 1 and fuse/relay box or a faulty junction block No. 1.

CHART A1-2 (PRIZM) - MIL WON'T FLASH ANY CODES, OR ON STEADY

When Data Link Connector (DLC) terminals TE1 and E1 are jumpered, Malfunction Indicator Light (MIL) will flash codes that are in ECM's memory.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for short to ground in Red/Yellow wire.
2. Checks for open in Blue/White wire (diagnostic request terminal).
3. Checks for open in Brown ground wire to DLC or a faulty ECM.

CODE 12 (PRIZM) - NO RPM SIGNAL

ECM monitors crank angle and camshaft positions, and supplies .7 volt to crank angle and Camshaft Position (CMP) sensors. As signal rotor rotates, voltage is toggled high and low. ECM determines crank angle and camshaft positions by toggled voltage input. Code 12 will set if ECM does not detect crank angle input while engine is cranking for 2 seconds or more, or ECM does not detect camshaft position input with engine speed 600-4000 RPM for 3 seconds or more.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faults in crank angle input low circuitry and ECM.
2. Checks for faults in crank angle input high circuitry and ECM.
3. Checks for faults in camshaft position input low circuitry and ECM.
4. Checks for faults in camshaft position input high circuitry and ECM. Also for a misadjusted signal rotor air gap.

CODE 13 (PRIZM) - NO RPM SIGNAL

ECM monitors crank angle and camshaft positions, and supplies .7 volt to crank angle and Camshaft Position (CMP) sensors. As signal rotor rotates, voltage is toggled high and low. ECM determines crank angle and camshaft positions by toggled voltage input. Code 13 will set if ECM does not detect crank angle input with engine speed greater than 1500 RPM for .3 second or more.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faults in crank angle input low circuitry and ECM.
2. Checks for faults in crank angle input high circuitry and ECM. Also for a misadjusted signal rotor air gap.

CODE 14 (PRIZM)

Igniter receives 5 volts from ECM. Igniter processes voltage then returns voltage to ECM as a ignition fail-safe signal. ECM also receives a .7 volt ignition signal when starter is being cranked. ECM uses these signals to pulse injectors. Code 14 will set if ignition fail-safe signal is not present 4 successive times during cranking.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for 5-volt signal from ECM.
2. Verifies .10-2 volt signal from ECM.
3. Checks for power to distributor.
4. Checks for open in Black/Orange wire between junction block No. 1 and distributor or a faulty junction block No. 1.
5. Checks for open in power feed to igniter, or a faulty ignition coil or igniter.
6. Checks for open in Red/Black or Black/Orange wires to ignition switch and a faulty ignition switch.

CODE 21 (PRIZM) - OXYGEN SENSOR, OPEN OR SHORTED CIRCUIT

Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 21 will set if sensor input voltage at ECM is .35-.70 volt for 60 seconds or more with engine speed greater than 1500 RPM, and coolant temperature is greater than 176°F (80°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sensor.
2. Checks for open in Black wire between ECM and sensor.
3. Checks for open in Brown wire between ECM and G106.
4. Checks for short to ground in Black wire between ECM and sensor, open in Brown wire to sensor (federal emissions only), and a faulty ECM.

CODE 22 (PRIZM) - ENGINE COOLANT TEMPERATURE SENSOR, OPEN OR SHORTED CIRCUIT

Engine Coolant Temperature (ECT) sensor is a thermistor in series with a fixed resistor in ECM. ECM applies 5 volts to sensor. ECM monitors voltage across ECT sensor and converts it into a temperature reading. When engine is cold, ECT sensor resistance is high. When engine is warm, ECT sensor resistance is low. Therefore, when engine is cold, ECM receives a high voltage input and when engine is warm, ECM receives a low voltage input. Code 22 will set if voltage input at ECM indicates either a very low or very high engine coolant temperature for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in White wire between ECM and ECT sensor or a faulty ECM.
2. Checks for open in Brown wire between ECM and ECT sensor or a faulty ECM.
3. Determines if problem is a faulty ECT sensor or ECM.

CODE 24 (PRIZM) - INTAKE AIR TEMPERATURE SENSOR, OPEN OR SHORTED CIRCUIT

Intake Air Temperature (IAT) sensor is a thermistor in series with a fixed resistor in ECM. IAT sensor measures temperature of air entering intake manifold. ECM applies 5 volts to IAT sensor. ECM monitors voltage across IAT sensor and converts it into temperature reading. When outside air temperature is cold, IAT resistance is high. When outside air temperature is warm, IAT resistance is low. Therefore, when air temperature is cold, ECM will receive a high voltage input. When air temperature is warm, ECM will receive a low voltage input. Code 24 will set if voltage input at ECM indicates either a very low or very high air temperature for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in Yellow/Black wire between ECM and IAT sensor, and a faulty ECM.
2. Checks for open in Brown wire between ECM and IAT sensor, and a faulty ECM.
3. Determines if problem is a faulty IAT sensor or ECM.

CODE 25 (PRIZM) - OXYGEN SENSOR, LEAN EXHAUST INDICATED

Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 25 will set if sensor input voltage at ECM is less than .45 volt for 90 seconds or more with sensor warm and engine speed greater than 1500 RPM, vehicle speed is less than 60 MPH, and engine coolant temperature is greater than 122°F (50°C). Code 25 will also set if engine speed varies more than 15 RPM over preceding crank angle period for 30 seconds while idling with engine coolant temperature greater than 122°F (50°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sensor.
2. Checks for open in Black wire between ECM and sensor.
3. Checks for open in Brown wire between ECM and G106.
4. Checks for short to ground in Black wire between ECM and sensor, open in Brown wire to sensor (federal emissions only), and a faulty ECM.

CODE 26 (PRIZM) - OXYGEN SENSOR, RICH EXHAUST INDICATED

Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 26 will set if engine speed varies more than 15 RPM over preceding crank angle period for 30 seconds while idling with engine coolant temperature greater than 122°F (50°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sensor.
2. Checks for open in Black wire between ECM and sensor.
3. Checks for open in Brown wire between ECM and G106.
4. Checks for short to ground in Black wire between ECM and sensor, open in Brown wire to sensor (Federal emissions only), and a faulty ECM.

CODE 27 (PRIZM - CALIF. EMISSIONS) - SUB-OXYGEN SENSOR, OPEN OR SHORTED CIRCUIT

Sub-oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust after converter. ECM uses this input and Oxygen (O2) sensor input, and adjusts fuel to maintain air/fuel ratio. Code 27 will set when engine is at operating temperature and engine is off idle for 2 seconds or more with sensor input at ECM .45 volt or more, and sub-O2 sensor input at ECM .45 volt or less.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sub-O2 sensor.
2. Checks for open in Black wire between ECM and sub-O2 sensor.
3. Checks for open in Brown wire between ECM and G106.
4. Checks for short to ground in Black wire between ECM and sub-O2 sensor, open in Brown wire to sub-O2 sensor, and a faulty ECM.

CODE 31 (PRIZM) - MANIFOLD ABSOLUTE PRESSURE SENSOR, OPEN OR SHORTED CIRCUIT

MAP sensor responds to changes in manifold pressure (vacuum). MAP sensor consists of a semi-conductor type pressure converting element which converts a pressure change into an electrical change and an electrical circuit which amplifies and corrects electrical change. ECM applies 5 volts to MAP sensor.

Changes in pressure, resulting from engine load and RPM, are converted into voltage input monitored by ECM. A low voltage input at ECM indicates low manifold pressure and a high voltage input at ECM indicates high manifold pressure. Code 31 will set if voltage input at ECM indicates either very low or very high manifold pressure for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in Yellow wire between ECM and MAP sensor.
2. Checks for open in Light Green/Red wire between ECM and MAP sensor.
3. Checks for open in Brown wire between ECM and MAP sensor, and a faulty ECM or MAP sensor.

CODE 41 (PRIZM) - THROTTLE POSITION SENSOR, OPEN OR SHORTED CIRCUIT

Throttle Position (TP) sensor consists of a contact point (idle switch) and a potentiometer, and is connected to throttle valve shaft on throttle body. TP sensor detects throttle valve opening. Idle switch detects throttle opening and turns on during idle. Fully opened throttle is detected by potentiometer.

ECM provides a 5-volt reference voltage to TP sensor and as brush moves over resistance according to throttle valve opening, output voltage varies accordingly. ECM monitors on/off signal and sensor output voltage, and uses these to detect throttle valve opening. Code 41 will set if throttle position input at ECM indicates Wide Open Throttle (WOT) with engine idling for at least .5 second, or if throttle position input at ECM indicates and idle signal with engine off idle for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for a fault in Yellow wire between ECM and TP sensor.
2. Checks for open in Brown wire between ECM and TP sensor.
3. Checks for a fault in Light Green wire between ECM and TP sensor, a faulty ECM, or a misadjusted or faulty TP sensor.

CODE 42 (PRIZM) - VEHICLE SPEED SENSOR CIRCUIT, OPEN OR SHORTED CIRCUIT

Vehicle Speed Sensor (VSS) is an electronic relay mounted on transaxle. As transaxle rotates VSS, VSS provides speedometer with a vehicle speed input (voltage pulses). This signal drives speedometer. Speedometer converts vehicle speed input into a more precise waveform and provides ECM, and cruise control module with a vehicle speed input. ECM converts this input (ground pulses) into vehicle speed. Code 42 will set if vehicle speed input is not sensed at ECM with engine speed 3000-5000 RPM and transaxle in gear, for at least 8 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if VSS is functioning properly.
2. Checks for open in power feed to VSS.
3. Checks for open or short in ECM speed input circuit or a faulty ECM.
4. Determines if ECM or speedometer is faulty.
5. Checks for open in ground to VSS.
6. Checks for open in power feed to speedometer.
7. Determines if VSS or speedometer is faulty.

CODE 43 (PRIZM) - CRANK SIGNAL, NO SIGNAL

ECM receives a signal when ignition switch is in START position. ECM uses this signal, along with others, to determine injection time of fuel injectors. If signal is not detected during ignition, ECM will still allow engine to start, but Code 43 will set. Code 43 will set when crank signal is not detected at ECM during ignition.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open in Black wire between Clutch Pedal Position (CPP) switch (M/T) or Park/Neutral Position (PNP) switch (A/T) and ECM, or a faulty ECM.

CODE 51 (PRIZM) - SWITCH CONDITION SIGNAL

ECM receives a signal when the following occurs

1. Accelerator pedal is depressed (indicating throttle lever is in off idle position).
2. Vehicle is equipped with A/C and A/C switch is depressed with blower speed selector in any position except OFF (indicating A/C operation has been requested).
3. Vehicle is equipped with A/T and selector lever is in Reverse or any forward gear (indicating transaxle is in gear other than Park or Neutral).

Code 51 indicates that above systems are functioning normally. If Code 51 is not indicated with accelerator depressed, A/C is on or with gear selector lever in Reverse or any forward gear (excluding Park or Neutral) with Data Link Connector (DLC) terminals TE2 and TE1 jumpered to ground, a fault is present. If Code 51 is indicated without the above conditions, a fault is also present.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if systems are functioning normally in test diagnostic mode.
2. Checks if TP sensor circuit is operating normally.
3. Checks for short to ground in A/C circuit, open in PNP switch circuit, open in TP sensor circuit, or a faulty ECM or A/C amplifier.
4. Checks for short to ground in TP sensor circuit, or a faulty TP sensor or ECM.
5. Checks for short to ground in PNP switch circuit.
6. Determines whether system is functioning normally or if there is a problem in A/C circuit.
7. Determines if A/C amplifier or ECM is faulty.

CODE 51 - Schematic (Prizm) Switch Condition Signal. Scheme 92

CODE 51 - Flow Chart (Prizm) (1 OF 4) Switch Condition Signal. Scheme 93

CODE 51 - Flow Chart (Prizm) (2 OF 4) Switch Condition Signal. Scheme 94

CODE 51 - Flow Chart (Prizm) (3 OF 4) Switch Condition Signal. Scheme 95

CODE 51 - Flow Chart (Prizm) (4 OF 4) Switch Condition Signal. Scheme 96

CODE 71 (PRIZM - CALIF. EMISSIONS) - EXHAUST GAS

RECIRCULATION (EGR) SYSTEM

Exhaust Gas Recirculation (EGR) temperature sensor is a thermistor in series with a fixed resistor in ECM. ECM applies 5 volts to sensor. ECM monitors voltage across EGR temperature sensor and converts it into a temperature reading. When exhaust gases are cold, EGR temperature sensor resistance is high. When exhaust gases are warm, EGR temperature sensor resistance is low. Therefore, when exhaust gases are cold, ECM receives a high voltage input and when exhaust gases are warm, ECM receives a low voltage input. EGR Solenoid Vacuum (SV) valve is also controlled by ECM. EGR SV valve permits exhaust gas to be recirculated and incorporated into combustion process. ECM by monitoring exhaust gas temperature will determine when to energize EGR SV valve. Code 71 will set if engine coolant temperature is greater than 140°F (60°C) with EGR temperature less than 158°F (70°C) for at least 50 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks power feed to sensor.
2. Checks ground circuit to sensor.
3. Checks EGR temperature resistance vs. temperature values.
4. Checks power feed circuit to EGR SV valve.
5. Checks for a faulty EGR SV valve, mechanical checks and a faulty ECM.

CHART A2-1 (PRIZM LSi) - NO MALFUNCTION INDICATOR LIGHT

Malfunction Indicator Light (MIL) should remain on when ignition switch is in ON position and engine is not running. Battery voltage is applied to indicator bulb. PCM controls indicator bulb and turns it on by providing a ground path through Red/Yellow wire to PCM.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks if engine will start.
2. Checks if PCM is receiving power.
3. Checks for open in Red/Yellow wire to PCM or a faulty PCM.
4. Checks if EFI F-HTR relay switch is receiving power.
5. Checks if PCM has a good ground.
6. Checks for open in Red/Yellow wire to instrument cluster.
7. Checks if MIL is receiving power.
8. Checks if EFI F-HTR relay coil is receiving power.
9. Checks for open in EFI F-HTR ground wire.
10. Checks for open in Black/Orange wire between junction block 1 and fuse/relay box or a faulty junction block 1.

CHART A2-2 (PRIZM LSi) - (MIL) WON'T FLASH ANY CODES, ON STEADY

When Data Link Connector (DLC) terminals TE1 and E1 are jumpered, Malfunction Indicator Light (MIL) will flash codes that are in PCM's memory.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for short to ground in Red/Yellow wire.
2. Checks for open in Blue/White wire (diagnostic request terminal).
3. Checks for open in Brown ground wire to DLC or a faulty PCM.

CODE 12 (PRIZM LSi) - NO RPM SIGNAL

PCM monitors crank angle and camshaft positions, and supplies .7 volt to crank angle and Camshaft Position (CMP) sensors. As signal rotor rotates, voltage is toggled high and low. PCM determines crank angle and camshaft positions by toggled voltage input. Code 12 will set if PCM does not detect crank angle input while engine is cranking for 2 seconds or more, or PCM does not detect camshaft position input with engine speed 600-4000 RPM for 3 seconds or more.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faults in crank angle input low circuitry and PCM.
2. Checks for faults in crank angle input high circuitry and PCM.
3. Checks for faults in camshaft position input low circuitry and PCM.
4. Checks for faults in camshaft position input high circuitry and PCM. Also for a misadjusted signal rotor air gap.

CODE 13 (PRIZM LSi) - NO RPM SIGNAL

PCM monitors crank angle and camshaft positions, and supplies .7 volt to crank angle and Camshaft Position (CMP) sensors. As signal rotor rotates, voltage is toggled high and low. PCM determines crank angle and camshaft positions by toggled voltage input. Code 13 will set if PCM does not detect crank angle input with engine speed greater than 1500 RPM for .3 second or more.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faults in crank angle input low circuitry and PCM.
2. Checks for faults in crank angle input high circuitry and PCM. Also for a misadjusted signal rotor air gap.

CODE 14 (PRIZM LSi) - NO IGNITION SIGNAL

Igniter receives 5 volts from PCM. Igniter processes voltage then returns voltage to PCM as a ignition fail-safe signal. PCM also receives a .7 volt ignition signal when starter is being cranked. PCM uses these signals to pulse injectors. Code 14 will set if ignition fail-safe signal is not present 4 successive times during cranking.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for 5-volt signal from PCM.
2. Verifies .10-2 volt signal from PCM.
3. Checks for power to distributor.
4. Checks for open in Black/Orange wire between junction block No. 1 and distributor or a faulty junction block No. 1.
5. Checks for open in power feed to igniter, or a faulty ignition coil or igniter.
6. Checks for open in Red/Black or Black/Orange wires to ignition switch and a faulty ignition switch.

CODE 16 (PRIZM LSi) - PCM CONTROL SIGNAL, PCM FAILURE

PCM has detected a failure in communication circuitry between electronic engine and electronic transaxle controls. PCM will no longer allow transaxle to go into a Torque Converter Clutch (TCC) lock-up condition. Replace PCM if Code 16 is set.

CODE 16 - Schematic (Prizm LSi) PCM Control Signal, PCM Failure. Scheme 97

CODE 21 (PRIZM LSi) - OXYGEN SENSOR CIRCUIT, OPEN OR SHORTED CIRCUIT

Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean)-1.0 (rich) volt. PCM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 21 will set if O2 sensor input voltage at PCM is .35-.70 volt for 60 seconds or more with engine speed greater than 1500 RPM, and coolant temperature is greater than 176°F (80°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty O2 sensor.
2. Checks for open in Black wire between PCM and O2 sensor.
3. Checks for open in Brown wire between PCM and G106.
4. Checks for short to ground in Black wire between PCM and O2 sensor, open in Brown wire to O2 sensor (federal emissions only), and a faulty PCM.

CODE 22 (PRIZM LSi) - ENGINE COOLANT TEMPERATURE SENSOR, OPEN OR SHORTED CIRCUIT

Engine Coolant Temperature (ECT) sensor is a thermistor in series with a fixed resistor in PCM. PCM applies 5 volts to sensor. PCM monitors voltage across ECT sensor and converts it into a temperature reading. When engine is cold, ECT sensor resistance is high. When engine is warm, ECT sensor resistance is low. Therefore, when engine is cold, PCM receives a high voltage input and when engine is warm, PCM receives a low voltage input. Code 22 will set if voltage input at PCM indicates either a very low or very high engine coolant temperature for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in White wire between PCM and ECT sensor or a faulty PCM.
2. Checks for open in Brown wire between PCM and ECT sensor or a faulty PCM.
3. Determines if problem is a faulty ECT sensor or PCM.

CODE 24 (PRIZM LSi) - INTAKE AIR TEMPERATURE SENSOR, OPEN OR SHORTED CIRCUIT

Intake Air Temperature (IAT) sensor is a thermistor in series with a fixed resistor in PCM. IAT sensor measures temperature of air entering intake manifold. PCM applies 5 volts to IAT sensor. PCM monitors voltage across IAT sensor and converts it into temperature reading. When outside air temperature is cold, IAT resistance is high. When outside air temperature is warm, IAT resistance is low. Therefore, when air temperature is cold, PCM will receive a high voltage input. When air temperature is warm, PCM will receive a low voltage input. Code 24 will set if voltage input at PCM indicates either a very low or very high air temperature for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in Yellow/Black wire between PCM and IAT sensor, and a faulty PCM.
2. Checks for open in Brown wire between PCM and IAT sensor, and a faulty PCM.
3. Determines if problem is a faulty IAT sensor or PCM.

CODE 25 (PRIZM LSi) - OXYGEN SENSOR, LEAN EXHAUST INDICATED

Oxygen sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean)-1.0 (rich) volt. PCM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 25 will set if O2 sensor input voltage at PCM is less than .45 volt for 90 seconds or more with O2 sensor warm and engine speed greater than 1500 RPM, vehicle speed is less than 60 MPH, and engine coolant temperature is greater than 122°F (50°C). Code 25 will also set if engine speed varies more than 15 RPM over preceding crank angle period for 30 seconds while idling with engine coolant temperature greater than 122°F (50°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sensor.
2. Checks for open in Black wire between PCM and sensor.
3. Checks for open in Brown wire between PCM and G106.
4. Checks for short to ground in Black wire between PCM and sensor, open in Brown wire to sensor (federal emissions), and a faulty PCM.

CODE 26 (PRIZM LSi) - OXYGEN SENSOR, RICH EXHAUST INDICATED

Oxygen sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. PCM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. Code 26 will set if engine speed varies more than 15 RPM over preceding crank angle period for 30 seconds while idling with engine coolant temperature greater than 122°F (50°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sensor.
2. Checks for open in Black wire between PCM and sensor.
3. Checks for open in Brown wire between PCM and G106.
4. Checks for short to ground in Black wire between PCM and sensor, open in Brown wire to sensor (Federal emissions), and a faulty PCM.

CODE 27 (PRIZM LSi - CALIF. EMISSIONS) - SUB-OXYGEN SENSOR, OPEN OR SHORTED CIRCUIT

Sub-oxygen sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. PCM monitors voltage and determines concentration of gases in exhaust after converter. PCM uses this input and Oxygen (O2) sensor input, and adjusts fuel to maintain air/fuel ratio. Code 27 will set when engine is at operating temperature and engine is off idle for 2 seconds or more with O2 sensor input at PCM .45 volt or more, and sub-O2 sensor input at PCM .45 volt or less.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for faulty sub-O2 sensor.
2. Checks for open in Black wire between PCM and sub-O2 sensor.
3. Checks for open in Brown wire between PCM and G106.
4. Checks for short to ground in Black wire between PCM and sub-O2 sensor, open in Brown wire to sub-O2 sensor, and a faulty PCM.

CODE 31 (PRIZM LSi) - MANIFOLD ABSOLUTE PRESSURE SENSOR, OPEN OR SHORTED CIRCUIT

MAP sensor responds to changes in intake manifold pressure (vacuum). MAP sensor consists of a semi-conductor type pressure converting element which converts a pressure change into an electrical change and an electrical circuit which amplifies and corrects electrical change. PCM applies 5 volts to MAP sensor. Changes in pressure, resulting from engine load and RPM, are converted into voltage input monitored by PCM. A low voltage input at PCM indicates low manifold pressure and a high voltage input at PCM indicates high manifold pressure. Code 31 will set if voltage input at PCM indicates either very low or very high manifold pressure for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open or short to ground in Yellow wire between PCM and MAP sensor.
2. Checks for open in Light Green/Red wire between PCM and MAP sensor.
3. Checks for open in Brown wire between PCM and MAP sensor, and a faulty PCM or MAP sensor.

CODE 41 (PRIZM LSi) - THROTTLE POSITION (TP) SENSOR, OPEN OR SHORTED CIRCUIT

Throttle Position (TP) sensor consists of a contact point (idle switch) and a potentiometer, and is connected to throttle valve shaft on throttle body. TP sensor detects throttle valve opening. Idle switch detects throttle opening and turns on during idle. Fully opened throttle is detected by potentiometer. PCM provides a 5-volt reference voltage to TP sensor and as brush moves over resistance according to throttle valve opening, output voltage varies accordingly. PCM monitors on/off signal and sensor output voltage, and uses these to detect throttle valve opening. Code 41 will set if throttle position input at PCM indicates Wide Open Throttle (WOT) with engine idling for at least .5 second, or if throttle position input at PCM indicates and idle signal with engine off idle for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for a fault in Yellow wire between PCM and TP sensor.
2. Checks for open in Brown wire between PCM and TP sensor.
3. Checks for a fault in Light Green wire between PCM and TP sensor, a faulty PCM, or a misadjusted or faulty TP sensor.

CODE 42 (PRIZM LSi) - VEHICLE SPEED SENSOR, OPEN OR SHORTED CIRCUIT

Vehicle Speed Sensor (VSS) is an electronic relay mounted on transaxle. As transaxle rotates VSS, VSS provides speedometer with a vehicle speed input (voltage pulses). This signal drives speedometer. Speedometer converts vehicle speed input into a more precise waveform and provides PCM, and cruise control module with a vehicle speed input. PCM converts this input (ground pulses) into vehicle speed. Code 42 will set if vehicle speed input is not sensed at PCM with engine speed 3000-5000 RPM and transaxle in gear, for at least 8 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if VSS is functioning properly.
2. Checks for open in power feed to VSS.
3. Checks for open or short in PCM speed input circuit or a faulty PCM.
4. Determines if PCM or speedometer is faulty.
5. Checks for open in ground to VSS.
6. Checks for open in power feed to speedometer.
7. Determines if VSS or speedometer is faulty.

CODE 43 (PRIZM LSi) - NO CRANK SIGNAL

PCM receives a signal when ignition switch is in START position. PCM uses this signal, along with others, to determine injection time of fuel injectors. If signal is not detected during ignition, PCM will still allow engine to start, but Code 43 will set. Code 43 will set when crank signal is not detected at PCM during ignition.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for open in Black wire between Clutch Pedal Position (CPP) switch (M/T) or Park/Neutral Position (PNP) switch (A/T) and PCM, or a faulty PCM.

CODE 51 (PRIZM LSi) - SWITCH CONDITION SIGNAL

PCM receives a signal when the following occurs

1. Accelerator pedal is depressed (indicating throttle lever is in off idle position).
2. Vehicle is equipped with A/C and A/C switch is depressed with blower speed selector in any position except OFF (indicating A/C operation has been requested).
3. Vehicle is equipped with A/T and selector lever is in Reverse or any forward gear (indicating transaxle is in gear other than Park or Neutral).

Code 51 indicates that above systems are functioning normally. If Code 51 is not indicated with accelerator depressed, A/C on or with gear selector lever in Reverse or any forward gear (excluding Park or Neutral) with Data Link Connector (DLC) terminals TE2 and TE1 jumpered to ground, a fault is present. If Code 51 is indicated without the above conditions, a fault is also present.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if systems are functioning normally in test diagnostic mode.
2. Checks if TP sensor circuit is operating normally.
3. Checks for short to ground in A/C circuit, open in PNP switch circuit, open in TP sensor circuit, or a faulty PCM or A/C amplifier.
4. Checks for short to ground in TP sensor circuit, or a faulty TP sensor or PCM.
5. Checks for short to ground in PNP switch circuit.
6. Determines whether system is functioning normally or if there is a problem in A/C circuit.
7. Determines if A/C amplifier or PCM is faulty.

CODE 51 - Schematic (Prizm LSi) Switch Condition Signal. Scheme 98

CODE 51 - Flow Chart (Prizm LSi) (1 OF 4) Switch Condition Signal. Scheme 99

CODE 51 - Flow Chart (Prizm LSi) (2 OF 4) Switch Condition Signal. Scheme 100

CODE 51 - Flow Chart (Prizm LSi) (3 OF 4) Switch Condition Signal. Scheme 101

CODE 51 - Flow Chart (Prizm LSi) (4 OF 4) Switch Condition Signal. Scheme 102

CODE 52 (PRIZM LSi) - KNOCK SENSOR, OPEN OR SHORTED CIRCUIT

Knock sensor (located in rear of engine block) is used to detect engine knock (abnormal vibration). PCM will retard ignition timing based on signal received from knock sensor when engine knock is present. Knock sensor is constructed of a piezoelectric element which generates an AC voltage when it becomes deformed (vibration or engine knock detected). Knock sensor sends this voltage signal to PCM which in turn retards ignition timing. Engine knock will no longer be detected with reduced ignition timing. Code 52 will set if an open or short in knock sensor circuit is present with engine speed 1200-6000 RPM for at least .5 second.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for possible shorts in knock sensor circuit.
2. Determines if knock sensor is faulty, Black wire is open or shorted, or PCM is faulty.

CODE 71 (PRIZM LSi - CALIF. EMISSIONS) - EXHAUST GAS RECIRCULATION (EGR) SYSTEM

Exhaust Gas Recirculation (EGR) temperature sensor is a thermistor in series with a fixed resistor in PCM. PCM applies 5 volts to sensor. PCM monitors voltage across EGR temperature sensor and converts it into a temperature reading. When exhaust gases are cold, EGR temperature sensor resistance is high. When exhaust gases are warm, EGR temperature sensor resistance is low. Therefore, when exhaust gases are cold, PCM receives a high voltage input and when exhaust gases are warm, PCM receives a low voltage input. EGR Solenoid Vacuum (SV) valve is also controlled by PCM. EGR SV valve permits exhaust gas to be recirculated and incorporated into combustion process. PCM by monitoring exhaust gas temperature will determine when to energize EGR SV valve. Code 71 will set if engine coolant temperature is greater than 140°F (60°C) with EGR temperature less than 158°F (70°C) for at least 50 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks power feed to sensor.
2. Checks ground circuit to sensor.
3. Checks EGR temperature resistance vs. temperature values.
4. Checks power feed circuit to EGR SV valve.
5. Checks for a faulty EGR SV valve, mechanical checks and a faulty PCM.

SUMMARY

If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, proceed to TESTS W/O CODES article in the ENGINE PERFORMANCE Section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.