Emission control
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
The ECU compares the oxygen sensors before and after the catalytic converter on bank 1 and concludes that the converter is not cleaning the exhaust as efficiently as it should.
Common causes
- Worn or failed catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before or around the sensors
- Engine misfire or rich/lean running that damaged the converter
- Contaminated converter (oil, coolant)
Symptoms
- Check engine light, often the only symptom
- Failed emissions test
- Reduced power or fuel economy if the converter is clogged
- Rotten-egg smell in some cases
How to check
- Fix any misfire and fuel trim codes first
- Check for exhaust leaks near the sensors
- Compare upstream and downstream oxygen sensor waveforms (the rear one should be steadier)
- Check converter temperature difference and backpressure; replace the converter only if the sensors are good
Can you keep driving?
Usually drivable, but fix the root cause: a damaged converter can clog and restrict the engine. Do not ignore a misfire if one is present.
Related codes
Frequently asked questions
- What does P0420 mean?
- The catalytic converter on bank 1 is not cleaning the exhaust as well as the ECU expects.
- Does P0420 always mean I need a new catalytic converter?
- No. A faulty rear oxygen sensor or an exhaust leak can cause the same code, so test those first.
General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.