Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1990CHEVY VAN G20, 4.3 Z, 475/M40REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - DIESELTROUBLE CODE IDENTIFICATIONNOTES
1990 Chevrolet Chevy Van G20, 4.3 Z, 475/M40
Trouble Code Identification: Notes
1990 Chevrolet Chevy Van G20, 4.3 Z, 475/M40SECTION Notes
- The DEC system codes indicate a failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of the ECM. Code 51 indicates that PROM is improperly installed or has failed.
- Code 52 indicates that the ECM has failed and must be replaced. If the ECM is replaced and the condition was not corrected, the following may be the cause:
- An incorrect ECM or PROM application may cause a malfunction and may, or may not, set a code.
- The ECM connector may be the problem. Connector terminals may have to be removed from the connector to properly check them.
- PROM failure. Although the PROM rarely fails, it could be the cause of the problem.
- Replacement ECM may be faulty.
- Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
- A shorted solenoid, coil relay, or harness may cause an ECM to fail and a replacement ECM to fail when it is installed. Use Short Circuit Tester (J-34636), to check for short circuits.
TROUBLE CODE IDENTIFICATION
| Code (Affected Circuit) | Probable Cause |
|---|---|
| 12 (Engine Speed Sensor) | No engine speed sensor reference pulse to ECM. |
| 14 (CTS High Temp.) | Sensor or signal line grounded. |
| 15 (CTS Low Temp.) | Sensor, connections, or wires open. |
| 21 (TPS Voltage High) | Open circuit or misadjusted TPS. |
| 22 (TPS Voltage Low) | Circuit grounded. |
| 23 (TPS Not Calibrated) | Voltage not .25-1.3 volts at curb idle. |
| 24 (Vehicle Speed Sensor) | VSS circuit open or grounded. |
| 31 (MAP Voltage Low) | Open or short to ground. |
| 32 (EGR Loop Error) | Improper EGR vacuum signal. |
| 33 (MAP Voltage High) | Open or short to ground. |
| 51 (PROM Fault) | Improperly installed or faulty PROM. |
| 52 (ECM Fault) | Fault in ECM circuit. |
| 53 (5-Volt Reference Overload) | Grounded 5-volt reference signal. |
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.