P0420 and the Catalytic Converter: Symptoms and Repair

P0420 (and P0430 on V engines) indicates the catalytic converter isn't meeting expected efficiency. But that doesn't always mean an expensive catalyst replacement is needed.

How the code sets

The ECU compares the oxygen sensor signals before and after the catalyst. If the downstream sensor starts to mirror the upstream sensor too closely, it concludes the catalyst is no longer processing exhaust gas well enough.

Before jumping to the catalyst

• Check the downstream oxygen (O2) sensor — a frequent culprit
• Look for an exhaust leak
• Check whether the engine burns oil or coolant (kills the catalyst)
• If there are misfire or rich/lean codes, fix those first

Is it really the catalyst?

If the above are clean and efficiency is still low, the catalyst may be aged. This is normal on high-mileage cars. It doesn't directly affect drive safety, but it causes emissions-test failures and gradual performance loss.

AutoMulti shows upstream and downstream oxygen sensor values live, helping you tell whether the problem is the sensor or the catalyst.