All codes › P0420

P0420 — Catalyst Efficiency Below Threshold

P0420 is set when the engine control module (ECM) compares the upstream and downstream oxygen sensor readings on Bank 1 and concludes the catalytic converter is no longer storing and releasing oxygen efficiently. In a healthy system, the downstream O2 sensor signal stays relatively flat compared to the switching upstream sensor. When both signals look similar, the ECM infers the catalyst has lost its ability to convert hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful byproducts.

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