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
PCM monitors the Heated Oxygen Sensor HO2S activity for 100 seconds. During this period, PCM counts the number of Lean/Rich (L/R) and Rich/Lean (R/L) switches. With this information, an average time for all transitions can be determined. PCM then divides R/L average by L/R rich average to obtain a ratio.
Conditions for setting DTC are: no active fuel trim, injector circuit, misfire, AIR, CKP, ECT, EGR, EVAP, IAT, MAF, MAP or TP sensor DTCs set. P0131, P0132 or P0135 not set, system voltage greater than 9 volts, system in closed-loop, engine running for at least 3.3 minutes, ECT greater than 167°F (75°C), engine speed 1000-3000 RPM, MAF 15-40 gm/s and these conditions stable for 3 seconds.
DTC will set when PCM detects degraded (slow) rich-to-lean or lean-to-rich response times for HO2S 1 or HO2S ratio is not within range.
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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.
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