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. VCM uses signal from Heated Oxygen Sensor (HO2S) behind TWC read efficiency of TWC. VCM will set DTC P0420 if TWC oxygen storage capacity is less than a predetermined threshold.
Conditions for setting DTC:
- System in "closed-loop".
- Commanded air/fuel ratio is 14.7:1.
- Engine run time equals 346 seconds.
- IAT 20-167°F (-6.75-75°C).
- ECT 167-243°F (75-117°C).
- BARO equals 73 kPa.
- Converter warm-up test passed.
- TP sensor not greater than 1.7 percent.
- Engine idle speed near specifications.
- VSS not greater than one MPH.
- No transmission related DTCs.
- No ECT, EGR, EVAP, HO2S, IAC, IC, IAT, MAF, MAP, TP or VSS sensor DTCs set.
- No fuel trim or misfire DTCs set.
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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.