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 PERFORMANCETESTING & DIAGNOSISENGINE CONTROLS - TESTS W/CODES - GASOLINETROUBLE CODE CHARTSCODE 13, OPEN OXYGEN SENSOR CIRCUIT (EXCEPT 4.3L TURBO)NOTES
1991 GMC Forward Control 5.7 K, Automatic
Code 13, Open Oxygen Sensor Circuit (Except 4.3L Turbo): Notes
1991 GMC Forward Control 5.7 K, AutomaticSECTION Notes
When exhaust temperature is less than 600°F (316°C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, Code 21 or 22 is not present, O2 signal voltage is steady at .35-.55 volt and throttle position sensor signal is greater than idle. All conditions must be met for at least 1 minute.
- This determines if fault is in O2 sensor, ECM or wiring.
- Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.
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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.