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.
Circuit Description
Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When vehicle is first started, the Powertrain Control Module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating air/fuel ratio. PCM supplies HO2S with a reference, or bias, voltage of about 450 mV. HO2S generates a voltage range of 0-1000 mV that fluctuates greater than, and less than bias voltage once in closed loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize time required for sensors to reach operating temperature, and to provide an accurate voltage signal. HO2S 2 is used for post catalyst monitoring. This DTC will set if HO2S 2 voltage remains at or near bias voltage amount.
Each HO2S 2 has the following circuits: High signal, low signal, heater ignition voltage and heater ground circuit.
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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.