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.

Real-world scan data

Seen in

17,645

scans in the past year

Found in

2.8%

of scans

5.0%

of scans that found faults

Appears alongside · share of P0420 scans

P0300in 16.8% of scans

P0301in 13.5% of scans

P0303in 13.1% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.