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 MANUALSGMC1991FORWARD CONTROL 5.7 K, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - GASOLINETROUBLE CODE CHARTSCODE 25, MAT SENSOR TEMP. HIGH (2.5L & 4.3L TURBO)DIAGNOSTIC AIDS
1991 GMC Forward Control 5.7 K, Automatic
Diagnostic Aids
1991 GMC Forward Control 5.7 K, AutomaticSECTION Diagnostic Aids
If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a Scan tester. A Code 25 will also result if sensor signal circuit is shorted to ground.
CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
| Application | ECM Terminal | Wire Color | |
|---|---|---|---|
| 2.5L | |||
| Signal | C12 | Tan | |
| MAT Ground | D2 | Black/Red | |
| 4.3L Turbo | |||
| Signal | GF16 | Tan | |
| MAT Ground | BB5 | Black | |
| °F (°C) | Ohms | ||||
|---|---|---|---|---|---|
| 210 (100) | 185 | ||||
| 160 (70) | 450 | ||||
| 100 (38) | 1800 | ||||
| 70 (20) | 3400 | ||||
| 40 (4) | 7500 | ||||
| 20 (-7) | 13,500 | ||||
| 0 (-18) | 25,000 | ||||
| -40 (-40) | 100,700 | ||||
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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.