P0420
Catalyst System Efficiency Below Threshold Bank 1
Auxiliary emissions control
Generic OBD-II
Medium
What it means
P0420 means the catalytic converter on Bank 1 isn't cleaning your exhaust as effectively as it should. The engine still runs, but it's putting out more pollutants than allowed — so you won't pass an emissions test until it's fixed.
Medium
Can I keep driving? Usually yes, short-term — but don't ignore it; a failing catalytic converter gets more expensive over time and won't pass an emissions test.
Common symptoms
What you'll notice
- Check-engine light on
- Slightly worse fuel economy
- Fails an emissions / smog test
- Occasional sulfur ("rotten egg") smell from the exhaust
Common causes
Most likely first
- Worn-out or failed catalytic converterLikely
- Faulty downstream oxygen (O2) sensorCommon
- Exhaust leak near the sensorsCommon
- Engine running too rich or too lean (a fuel or ignition fault)Less often
How to diagnose & fix
Work through it in order
- Read and confirm P0420, and fix any other trouble codes first (they can trigger a false P0420).
- Check the oxygen-sensor live data: a lazy downstream sensor that mirrors the upstream one points to the sensor, not the converter.
- Inspect for exhaust leaks around the converter and sensors.
- If the converter is genuinely worn out, replace it, then clear the code and retest.
FAQ
Quick answers
Is it safe to drive with P0420?+
Usually yes short-term — but it won't pass an emissions test and the condition can get worse.
Can I just clear the code?+
You can clear it, but if the fault is real it will come back within a few drive cycles.
Related codes
Often seen with P0420
Read & clear P0420 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX