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.
DTC P0420: Catalytic System Efficiency Below Threshold
- If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot those DTCs first, then recheck for DTC P0420.
P0137, P0138: Secondary HO2S (Sensor 2}
P0141: Secondary HO2S (Sensor 2) heater
- Before you troubleshoot, record all freeze data and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- Reset the ECM with the HDS (see HDS CLEAR COMMAND ).
- Start the engine. Hold the engine speed at 3,000 RPM without load (in Park or neutral) until the radiator fan comes on.
- Test-drive at 40-55 mph (64-88 km/h) for about 2 minutes. Try to keep the engine speed between 1,500 and 2,500 rpm. Then decelerate for at least 3 seconds with the throttle completely closed. Reduce the vehicle speed to 56 km/h (35 mph), and try to hold it until the HDS indicates catalyst readiness is set to complete.
Is the test result FAIL?
- YES -Check for cracks or melting in the three way catalytic converter (TWC). If it is damaged, replace the TWC.
- NO -Intermittent failure, system is OK at this time.
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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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