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
Three-Way Catalyst (TWC) system is used to control emission. PCM uses signal from Heated Oxygen Sensors (HO2S) to read efficiency of TWC. PCM will set DTC P0420 if TWC oxygen storage capacity is less than a predetermined threshold.
Conditions required to set DTC are:
- No CKP, ECT, HO2S, IAT, MAF, MAP, or TP sensor DTCs set.
- No EGR pintle position, EVAP purge, fuel trim, injector circuit, misfire or VSS DTCs set.
- Engine is operating in closed loop.
- Engine air load in below 99 percent.
- ECT is above 140°F (60°C).
- MAF is between 8-50 grams/second.
- Change in engine load is below eight percent.
- TWC temperature is above 750°F (399°C).
- Engine speed below 3500 RPM.
- Vehicle speed 16-75 MPH (26-123 km/h).
- PCM determines TWC oxygen storage capacity less than predetermined threshold.
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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.