Engine Controls - Tests W/codes Geo Prizm pre-I

INTRODUCTION

If no faults were found while performing BASIC TESTING , proceed with self-diagnostics. If no fault codes or only pass codes are present, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.).

CHECK ENGINE LIGHT

All vehicles are equipped with a CHECK ENGINE light, located on the instrument panel. The light will illuminate when the ignition switch is turned to the ON position (bulb check) and engine is not running. Light should not flash at this time and should go out when engine is started.

When the CHECK ENGINE engine light remains on or flashes, with the engine running, the self-diagnostic system has detected a problem. If the problem goes away, the light will go out after 10 seconds, but a trouble code will remain stored in the ECM memory.

Hard Failures

Hard failures will cause CHECK ENGINE light to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control unit 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 may cause CHECK ENGINE light to flicker or illuminate and go out after the intermittent fault goes away. The corresponding trouble code; however, will be retained in control unit memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from control unit memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article.

RETRIEVING CODES

1. Perform «DIAGNOSTIC CIRCUIT CHECK»(/geo/prizm/pre-i-1989-1991/remont/testing-diagnostics/#engine-controls-tests-wcodes) before diagnosing vehicle using self-diagnostic system. See «DIAGNOSTIC CIRCUIT CHECK»(/geo/prizm/pre-i-1989-1991/remont/testing-diagnostics/#engine-controls-tests-wcodes) chart in this article.
2. Ensure ignition switch is in the OFF position. Using a jumper wire, ground diagnostic connector terminal to ground terminal. (Scheme 5) Connector is located in the engine compartment. Turn ignition switch to the ON position. Read trouble codes.
3. If no trouble codes are stored, CHECK ENGINE light will flash rapidly on then off until jumper wire is disconnected. Count the number of flashes and look for pauses in flashing to ensure no trouble codes are overlooked. If trouble codes are stored, code will flash 3 times and the lowest code will be given first.

Scheme 5

TROUBLE CODE IDENTIFICATION CHART

CLEARING CODES

Turn ignition switch to OFF position. Remove STOP fuse from fuse block or negative battery terminal for at least 30 seconds. ECM trouble code memory is now clear.

ECM LOCATIONS

ECM LOCATION

Note. The following diagnostic flow charts and mini-schematics are courtesy of General Motors Corp.

Diagnostic Circuit Check Flow Chart. Scheme 6

CHART A-1, NO CHECK ENGINE LIGHT

With ignition switch in the ON position and engine off, CHECK ENGINE light should always be on. The ECM controls the CHECK ENGINE light by completing bulb ground circuit.

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

1. An open GAUGE fuse could indicate a short to ground in GAUGE fuse circuit.
2. This indicates a ground path through the ECM.

Diagnostic Aids

Check for a faulty light bulb, open in Red wire circuit or MAP sensor 5-volt reference shorted to ground. If engine cranks, but won't run, check battery feed circuit, EFI main relay, ignition circuit and poor ECM connections.

Chart A-1 Circuit Diagram & Flow Chart. Scheme 7

CHART A-2, CHECK ENGINE LIGHT WON'T FLASH ON STEADY

With ignition switch in the ON position and engine not running, CHECK ENGINE light should always be on. With check connector terminal grounded, the CHECK ENGINE light will flash on and off rapidly, or will flash any stored trouble codes. A steady CHECK ENGINE light suggests a short to ground or an open diagnostic terminal.

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

1. This checks for a grounded light circuit.
2. This determines if it is an open check connector terminal or a faulty ECM.

Chart A-2 VIN 5 Circuit Diagram. Scheme 8

Chart A-2 VIN 6 Circuit Diagram. Scheme 9

Chart A-2 Flow Chart. Scheme 10

CODES 12 & 13, NO RPM SIGNAL NO SIGNAL FOR 2 SECONDS

The ECM supplies 12 volts to crank angle sensor. As the stator inside the distributor passes the crank angle sensor, 12 volts is pulled low. This signal voltage is used to fire the coil and fuel injectors. If this signal is not present for 2 seconds, Codes 12 and 13 set.

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

1. Voltage is supplied by the ECM to the RPM pick-up coil.
2. Voltage is supplied by the ECM to the crank angle pick-up coil.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article.

Codes 12 & 13 VIN 5 Circuit Diagram. Scheme 11

Codes 12 & 13 VIN 6 Circuit Diagram. Scheme 12

Codes 12 & 13 Flow Chart. Scheme 13

CODE 14, IGNITION SIGNAL CIRCUIT NO SIGNAL (VIN 5)

The ECM receives a 12-volt signal from the ignitor on ECM terminal IGF. If this signal is not present 4 times during cranking, Code 14 will set. The ECM uses this signal to pulse the fuel injector.

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

1. This verifies condition of IGT circuit.
2. This verifies condition of IGF circuit.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article.

Code 14 VIN 5 Circuit Diagram. Scheme 14

Code 14 VIN 5 Flow Chart. Scheme 15

CODE 14, IGNITION SIGNAL CIRCUIT NO SIGNAL (VIN 6)

The ECM receives a 12-volt signal from the ignitor on ECM terminal IGF. If this signal is not present 4 times during cranking, Code 14 will set. The ECM uses this signal to pulse the fuel injector.

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

1. This verifies B+ signal from the ECM.
2. This checks for a short to ground in the Black wire.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article.

Code 14 VIN 6 Circuit Diagram. Scheme 16

Code 14 VIN 6 Flow Chart. Scheme 17

CODE 21, OXYGEN SENSOR CIRCUIT OPEN OR SHORTED CIRCUIT

The ECM supplies approximately .45 volt between ECM terminals OX+ and OX1 (VIN 5) or OX and E1 (VIN 6). If reading is taken with a 10-megohm DVOM, reading may be as low as .32 volt. The O2 sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .10-.90 volt.

The O2 sensor does not produce a voltage until sensor reaches 600°F (316°C). When sensor temperature is less than 600°F (316°C), an Open Loop condition exists.

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

1. Code 21 will set when engine is at normal operating temperature, at least 2 minutes have passed since engine start, O2 signal voltage is steady between 0-1 volt and all these conditions are present for 60 seconds.
2. This determines if O2 sensor is problem or if ECM and wiring are faulty.
3. When doing this check, use a 10-megohm DVOM. This checks continuity of Black and Brown wires. If Brown wire is open, ECM voltage on Black wire will be greater than .6 volt.

Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 sets in one minute if sensor signal voltage remains between 0-1 volt. Verify clean tight connection on Black wire (VIN 5) or Brown wire (VIN 6). An open Red or Black wire (VIN 5) or Black or Brown wire (VIN 6), will result in Code 21. If code is intermittent, see TESTS W/O CODES article.

Code 21 VIN 5 Circuit Diagram. Scheme 18

Code 21 VIN 6 Circuit Diagram. Scheme 19

Code 21 Flow Chart. Scheme 20

CODE 22, COOLANT TEMPERATURE SENSOR CIRCUIT OPEN OR SHORTED CIRCUIT

The Coolant Temperature Sensor (CTS) uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on White wire to the sensor. When engine is cold, thermistor resistance is high, this means the ECM will see high signal voltage.

As engine warms, thermistor resistance is less, this means the ECM will see low signal voltage. When engine reaches normal operating temperature, signal voltage will be approximately 1.5-2.0 volts at the ECM.

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

1. This verifies 4-5 volts is present at CTS connector.
2. This checks ground side of CTS circuit.
3. This determines if problem is caused by an open Brown wire or the ECM. Ensure ECM connectors properly fit.
4. This determines if circuit or ECM is the problem. Ensure ECM connectors fit properly.

After engine is started, engine coolant temperature should rise to approximately 203°F (95°C) and stabilize when thermostat opens. Code 22 will set if Brown or White wire is open. If code is intermittent, see TESTS W/O CODES article.

Code 22 Circuit Diagram & Flow Chart. Scheme 21

CODE 24, MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT LOW TEMPERATURE INDICATED

The MAT sensor uses a thermistor to control the signal voltage at the ECM. The ECM applies 4-6 volts to the sensor. When air is cold, thermistor resistance is high, so ECM sees a high signal voltage. If air is warm, sensor resistance is low, so ECM sees a low voltage.

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

1. This verifies 4-5 volts are present at MAT sensor connector.
2. This checks ground side of MAT sensor circuit.
3. This determines if problem is caused by an open circuit or ECM.
4. This determines if Black wire (VIN 5) or Yellow/Black wire (VIN 6) is open or if ECM is defective.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out terminals or damaged terminals. If code is intermittent, see TESTS W/O CODES article.

Code 24 Circuit Diagram. Scheme 22

Code 24 Flow Chart. Scheme 23

CODE 25, OXYGEN SENSOR CIRCUIT LEAN EXHAUST INDICATED

The ECM supplies approximately .45 volt between ECM terminals OX+ and OX1 (VIN 5) or OX and E1 (VIN 6). If reading is taken with a 10-megohm DVOM, reading may be as low as .32 volt. The O2 sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .10-.90 volt.

The O2 sensor does not produce a voltage until sensor reaches 600°F (316°C). When sensor is less than this temperature, an open loop condition exists.

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

1. Code 25 sets when O2 sensor signal voltage on Black wire remains below .2 volt for one minute or more with system in closed loop.

Check O2 sensor wire for contact with exhaust manifold. Check O2 wire for ground between connector and sensor. Check for fuel contamination. A small amount of water in fuel tank can be delivered to fuel injector causing Code 25 to set.

Check fuel pressure, see BASIC TESTING article. Check for exhaust leaks. An exhaust leak can cause outside air to be pulled into the exhaust past the O2 sensor. Vacuum or crankcase leaks can cause a lean condition. If Code 25 is intermittent, see TESTS W/O CODES article.

Code 25 VIN 5 Circuit Diagram. Scheme 24

Code 25 Flow Chart. Scheme 25

CODE 26, OXYGEN SENSOR CIRCUIT RICH EXHAUST INDICATED

The ECM supplies approximately .45 volt between ECM terminals OX+ and OX1 (VIN 5) or OX and E1 (VIN 6). If reading is taken with a 10-megohm DVOM, reading may be as low as .32 volt. The O2 sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .10-.90 volt.

The O2 sensor does not produce a voltage until sensor reaches 600°F (316°C). When sensor is less than this temperature, an open loop condition exists.

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

1. Code 26 will set when oxygen sensor signal voltage on Red wire (VIN 5) or Black wire (VIN 6) remains greater than .7 volt for 30 seconds or more with system in Closed Loop, engine run time after start is one minute or more and throttle angle is 3-45 percent.

If fuel system pressure goes high, the ECM can compensate for some increase, but if fuel pressure goes too high, Code 26 will set. See BASIC TESTING article. A leaking fuel injector can cause this code to set, see BASIC TESTING article.

A faulty canister purge system can cause Code 26, see I - SYS/COMP TESTS article. A faulty MAP sensor can cause this code to set. Disconnect MAP sensor connector. This allows the ECM to substitute a fixed MAP sensor signal value. If rich condition is gone, a faulty MAP sensor is causing rich condition.

An intermittent throttle switch output will cause the system to operate rich due to a false signal of engine acceleration. Inspect O2 sensor for silicone contamination from fuel or improper use of RTV sealant. The sensor may have White-powdery coating which can result in a high but false signal voltage (rich condition). Check for a sticking EGR valve at idle. If Code 26 is intermittent, see TESTS W/O CODES article.

Code 26 (VIN 5) Circuit Diagram. Scheme 26

Code 26 Flow Chart. Scheme 27

CODE 27, SUB-OXYGEN SENSOR OPEN OR SHORTED CIRCUIT VIN 5-CALIF. ONLY)

The ECM supplies a voltage of .45 volt between terminals OX2 and OX1. If reading is taken with a 10-megohm DVOM, reading may be as low as .32 volt. When the sub-oxygen sensor reaches operating temperature, it produces a varying voltage from about .1-.9 volt.

The sub-oxygen sensor produces no voltage when it is less than 600°F (316°F). An open sensor circuit, or cold sensor, causes Open Loop operation.

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

1. A Code 27 will set when engine is at normal operating temperature, running for at least 2 minutes, O2 sensor voltage is not varying and above conditions are met for about 60 seconds.
2. This determines if sub-O2 sensor is the problem or if the ECM and wiring are the problem.
3. This checks continuity of White and Black wires from ECM. If Black wire is open, the ECM voltage on the White wire will be greater than .6 volt (600 mV).

Ensure sub-O2 sensor has a clean, tight connection to Black wire. Open in the Black or White wire will result in a Code 27. If Code 27 is intermittent, see TESTS W/O CODES article.

Code 27 Circuit Diagram. Scheme 28

Code 27 Flow Chart. Scheme 29

CODE 31, MASS AIRFLOW SENSOR CIRCUIT SHORTED CIRCUIT VIN 5)

The Mass Airflow (MAF) sensor measures air passing through it in a given time. The ECM uses this information to monitor engine operating condition in calculating fuel delivery. A large quantity of air movement indicates acceleration, while a small quantity indicates deceleration or idle.

The MAF sensor produces a frequency signal which cannot be easily measured. The MAF sensor also contains the Manifold Air Temperature (MAT) sensor and provides ground to the circuit opening relay to run fuel pump once engine has started.

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

1. Verifies B+ and ground at MAF sensor.
2. Verifies proper voltage control to ECM from MAP sensor.
3. Verifies proper voltage from MAF sensor with engine RPM.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or broken wire inside the insulation. Inspect ECM harness connectors for backed out terminals. Check for damaged MAF housing.

Code 31 (VIN 5) Circuit Diagram. Scheme 30

Code 31 (VIN 5) Flow Chart. Scheme 31

CODE 31, MANIFOLD ABSOLUTE PRESSURE SENSOR OPEN OR SHORTED CIRCUIT (VIN 6)

The MAP sensor responds to changes in manifold pressure. The ECM receives a signal voltage that varies approximately 1-1.5 volts at closed throttle idle, to approximately 4-4.5 volts at wide open throttle (low vacuum). If MAP sensor fails, ECM will substitute a fixed MAP value and use TPS to control fuel delivery.

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

1. This verifies 5-volt reference is available from the ECM.
2. This step verifies ECM is grounding the circuit.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, see TESTS W/O CODES article.

Code 31 (VIN 6) Circuit Diagram & Flow Chart. Scheme 32

CODE 41, THROTTLE POSITION SENSOR OPEN OR SHORTED CIRCUIT VIN 5)

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve angle. Signal voltage varies from less than .60 volt at idle to about 4.5 volts at Wide Open Throttle (WOT).

The TPS provides the ECM with information used to decide closed Loop operation and indicates the off-idle condition to the ECM. The TPS is normally closed at idle and opens when engine speed is greater than 1800 RPM, which is then the off-idle condition. TPS information is used by the ECM for fuel control and many other ECM controlled outputs.

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

1. Voltage should be less than .6 volt at idle.
2. Voltage should increase to greater than 4.5 volts at WOT.

An intermittent may be caused by poor connection, rubbed through wire insulation or wire broken inside insulation. Inspect ECM harness connector for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness. If code is intermittent, see TESTS W/O CODES article.

Code 41 (VIN 5) Circuit Diagram. Scheme 33

Code 41 (VIN 5) Flow Chart. Scheme 34

CODE 41, THROTTLE SWITCH CIRCUIT (VIN 6)

On vehicles equipped with manual transaxle, the ECM supplies 4-5 volts to a throttle switch on the throttle body. When engine is idling, the ECM senses a low signal voltage, with slight throttle, ECM will sense a high signal voltage. At wide open throttle, signal voltage is low. Code 41 sets if ECM sees a low or high voltage signal all the time.

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

1. The ECM supplies battery voltage to the throttle switch. Poor connection or a ground circuit will indicate a low signal.
2. This checks for a misadjusted throttle switch.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, see TESTS W/O CODES article.

Code 41 (VIN 6) Circuit Diagram & Flow Chart. Scheme 35

CODE 42, VEHICLE SPEED SENSOR CIRCUIT

The ECM provides a 12-volt signal to the Vehicle Speed Sensor (VSS). The VSS will toggle the signal low and high indicating vehicle is moving. Code 42 sets if signal line stays high or low for 4 seconds or more.

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

1. If speedometer is inoperative, it could be the cause of Code 42.
2. The ECM supplies 12 volts to the VSS.
3. The VSS on the back of the instrument panel will toggle the 12 volts supplies by the ECM.

An intermittent may be caused by rubbed through insulation or a wire broken inside the wire insulation. Check ECM harness connectors for backed out or damaged terminals. If code is intermittent, see TESTS W/O CODES article. An open vacuum line can cause Code 42 to set. A careful visual inspection for air leaks should be made if vehicle speed signal circuit checks okay.

Code 42 Circuit Diagram & Flow Chart. Scheme 36

CODE 43, NO CRANK SIGNAL FROM STARTER

The ECM receives a 12-volt signal when starter is cranking. This signal is used by the ECM to know when to turn the fuel pump on. If signal is not present and engine speed is greater than 800 RPM, Code 43 will set.

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

1. Battery voltage should be present on the Black wire while engine is cranking.
2. When ignition switch is released (engine running), no voltage should be present on Black wire.

Code 43 Circuit Diagram & Flow Chart. Scheme 37

CODE 51, NO A/C SIGNAL

The A/C amplifier supplies a signal to the ECM when A/C is selected. This signal allows the ECM to adjust engine speed to compensate for additional load on engine. Code 51 will set if this signal is not present.

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

1. This determines that a signal is being sent from the A/C switch.
2. This determines that A/C amplifier is sending a signal to the ECM (VIN 5) or if A/C amplifier is sending signal to the A/C clutch relay (VIN 6).

If code is intermittent, see TESTS W/O CODES article.

Code 51 (VIN 5) Circuit Diagram. Scheme 38

Code 51 (VIN 6) Circuit Diagram. Scheme 39

Code 51 Flow Chart. Scheme 40

CODE 52, KNOCK SENSOR SIGNAL OPEN OR SHORTED CIRCUIT VIN 5)

The knock sensor is used to detect engine detonation. The ECM will retard the electronic spark timing based on signal being received. The knock sensor sends an AC voltage signal to the ECM through the Black wire and should measure about 2-300 mV AC. The amplitude and signal frequency are dependent upon the knock level.

If the Black wire becomes open or shorted to ground, voltage will drop to less than 200 mV. If this condition is met for about 5 seconds, Code 52 will be stored.

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

1. Code 52 will set when the following conditions are met for 5 seconds. Coolant temperature is above 194°F (90°C). MAT temperature is above 32°F (0°C). There is high engine load based on MAF and RPM sensors. Voltage on Black wire is less than 200 mV.

Repair internal engine problem if knock is detected at idle.

1. If tapping on engine lift hook does not produce a knock signal, try tapping closer to sensor before proceeding.
2. This checks the resistance of the knock sensor.

If code is intermittent, see TESTS W/O CODES article.

Code 52 Circuit Diagram. Scheme 41

Code 52 Flow Chart. Scheme 42

CODE 53, KNOCK CONTROL SIGNAL FAULTY ECM (VIN 5)

Check that all ECM connector's are fully seated into the ECM. If OK, replace ECM. Run engine and recheck for code.

CODE 71, EGR GAS TEMPERATURE SENSOR CIRCUIT (CALIFORNIA ONLY)

Code 71 indicates that EGR gas temperature sensor has signaled the ECM of a temperature greater than or less than the calibrated range.

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

1. The ECM should provide 4-5 volts to the EGR gas temperature sensor.
2. The ECM provides ground for the EGR gas temperature sensor.

Code 71 Circuit Diagram. Scheme 43

Code 71 Flow Chart. Scheme 44

SUMMARY

If no hard fault codes (or only pass codes) are present, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

See also:
• DIAGNOSTIC CIRCUIT CHECK