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: Catalyst System Efficiency Below Threshold (Bank 1): General Description
The PCM monitors exhaust emissions by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the 3-way catalytic converter (TWC). This HO2S #2 produces an output signal that the PCM uses to calculate the oxygen storage capacity of the catalyst. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat up, and then wait for a stabilization period while the engine is idling. The PCM then adds and removes fuel while monitoring the HO2S #2. When the catalyst is functioning properly, the HO2S #2 responses to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable 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.