Powertrain fault code
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
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Usually drivable. Fix any misfires first: they are often what kills the converter.
What P0420 means
The catalytic converter is not cleaning the exhaust as well as it should, judged by comparing the oxygen sensors before and after it.
Common causes
- Worn catalytic converter
- Faulty rear O2 sensor
- Exhaust leak
- Earlier misfires that damaged the converter
What to check first
- Check for exhaust leaks.
- Fix any misfire or fuel trim codes first.
- Have the rear O2 sensor checked before replacing the converter.
P0420 on popular makes
P0420 on a Honda
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- Check for exhaust leaks before the rear oxygen sensor: a leak can fake a P0420.
P0420 on a Nissan
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- Exhaust leaks at the manifold or flex pipe are a common cause of a false reading.
P0420 on a Chevrolet
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- Compare front and rear oxygen sensor readings: a rear sensor that switches like the front one means the cat is not storing oxygen.
P0420 on a Toyota
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- A lazy rear oxygen sensor can trigger it; check its live reading before replacing the catalytic converter.
P0420 on a Ford
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- Check for exhaust leaks and a slow rear oxygen sensor before buying a catalytic converter.
P0420 on a Subaru
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- P0420 is commonly reported on older, high-mileage Subarus; an aging catalytic converter is the usual cause once leaks and sensors check out.
P0420 on a Hyundai
- Fix any misfire or lean codes first: they overheat and damage the catalytic converter, and the new cat would fail too.
- Check the rear oxygen sensor and for exhaust leaks before replacing the catalytic converter.
Read your car's codes yourself, free
Plug a cheap ELM327 OBD-II adapter into the port under the dashboard and open the scan tool: Chrome or Edge on a computer with a USB adapter, or Chrome on Android with a Bluetooth LE adapter. It reads and clears codes and shows emissions readiness. It does not work on iPhone.
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General guidance only, not a diagnosis. A qualified mechanic should confirm the cause before you replace parts.