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.
Heated Oxygen Sensor
The HO2S produces a voltage that varies between 0.1-0.9 volt under normal operating conditions. The Powertrain Control Module (PCM) monitors this voltage and determines if the exhaust is lean or rich. If the voltage input at the PCM is about 0.1 volt the exhaust is lean, and if the voltage input is about 0.9 volt, the exhaust is rich. The PCM constantly monitors the HO2S 1 signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary. Heated Oxygen Sensors (HO2S) are installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the PCM to determine the performance of the TWC catalyst. For more information, see CATALYST MONITOR DIAGNOSTIC OPERATION under SELF-DIAGNOSTIC SYSTEM.
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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.