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 MANUALSGMC1991C3500 HD 6.2 J, STANDARD, NV4500/MT8REPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - SYSTEM/COMPONENT TESTSCOMPUTERIZED ENGINE CONTROLSENGINE SENSORS & SWITCHESOXYGEN (O2) SENSOR
1991 GMC C3500 HD 6.2 J, Standard, NV4500/MT8
Oxygen (O2) Sensor
1991 GMC C3500 HD 6.2 J, Standard, NV4500/MT8SECTION Oxygen (O2) Sensor
- Start engine and warm to operating temperature. Disconnect oxygen sensor. Connect a DVOM between Purple lead of oxygen sensor and ground. Place meter on the 2-volt scale.
- Using another DVOM on the 20-volt scale, connect voltmeter in series between Purple wire from the ECM and the positive post of battery. This will simulate a rich condition, causing ECM to respond by leaning mixture. Reading on voltmeter connected to oxygen sensor should decrease to a low voltage (less than .3 volt).
- Move voltmeter lead from positive battery post to negative battery post. This will simulate a lean condition, causing ECM to respond by richening mixture. Reading on voltmeter connected to oxygen sensor should increase (greater than .8 volt). If reading does not change as specified, replace O2 sensor.
- If a second DVOM is not available, install short jumper in Purple wire from the ECM. Hold jumper in one hand and touch positive post of battery with other hand to simulate a rich condition. Touch negative post of battery to simulate a lean condition. For additional testing procedures, see appropriate article below:
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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.