P0420 is one of the most common — and most misdiagnosed — check engine codes. It does not always mean a new cat.
The code blames the cat; the cause is often upstream
P0420 means the downstream oxygen sensor sees the converter not cleaning exhaust like it should. Sometimes the converter really is worn out. Just as often, a lazy O2 sensor, an exhaust leak before the sensor, or an engine running rich is skewing the reading. A $1,500 converter installed to fix a $90 sensor is the classic misdiagnosis — test first, replace second.
Quick tips
- Ask for sensor waveform data before approving a converter
- A rotten-egg smell alongside P0420 strengthens the case for a real cat failure
- Driving with P0420 is usually safe short-term but can mask new codes
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