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 MANUALSCHEVROLET1990FORWARD CONTROL 7.4 N, AUTOMATIC, 475/M40REPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - DIESEL
1990 Chevrolet Forward Control 7.4 N, Automatic, 475/M40
Engine Controls - Tests W/Codes - Diesel
1990 Chevrolet Forward Control 7.4 N, Automatic, 475/M40SECTION Engine Controls - Tests W/Codes - Diesel
- Introduction
- Self-Diagnostic System
- Diagnostic Procedure
- Entering Or Exiting Diagnostic Mode
- Trouble Code Identification
- ECM Location
- Diagnostic Charts
- Notes
- Diagnostic Circuit Check
- Service Engine Soon Light Inoperative
- ECM Check, Service Engine Soon Light On At All Times Or Won't Flash Code 12
- EPR Solenoid Electrical Check
- EPR Vacuum Circuit Check
- Code 12, No Reference Pulse
- Code 14, Coolant Temp Sensor Signal Voltage Low
- Code 15, Coolant Temp Sensor Signal Voltage High
- Code 21, Throttle Position Sensor Signal Voltage High
- Code 22, Throttle Position Sensor Signal Voltage Low
- Code 23, Throttle Position Sensor MISADJUSTED
- Code 24, Vehicle Speed Sensor Circuit
- Code 31, Manifold Absolute Pressure Sensor Signal Voltage Low
- Code 32, EGR Circuit Loop Error
- Code 33, Map Sensor Signal Voltage High
- Code 51, Prom Problem
- Code 52, ECM Fault
- Code 53, Voltage Reference Overload
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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.