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

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.

P0420 by make

Based on commonly reported failure patterns across owner and repair communities, not on our scan data.

Chevrolet

On higher-mileage Chevrolet cars and trucks this is often real converter wear, but exhaust leaks near the manifold and a lazy rear oxygen sensor can look identical. Clear up any misfire codes before replacing the converter.

Ford

Often seen on high-mileage Ford trucks and SUVs as the converter wears out, though a tired rear oxygen sensor or an exhaust leak can set the same code. Fix any misfire or fuel-trim codes first, since they shorten converter life.

Honda

Commonly reported on Hondas such as the Accord and Civic, where an aging front air-fuel sensor reacts slowly and can mimic a worn converter. Have the sensor readings checked before paying for a new converter.

Subaru

Commonly reported on Subarus as the converter ages, and sometimes after oil burning or a head gasket leak has coated the catalyst. If you also notice coolant loss or oil use, deal with that first or a new converter may fail the same way.

Toyota

On higher-mileage Toyotas such as the Corolla and Camry, this is often genuine converter wear rather than a bad sensor, especially if the converter has never been replaced. Check for exhaust leaks and any oil burning first, since those can ruin a new converter quickly.

Volkswagen

Commonly reported on turbocharged VW 2.0T models alongside oil-burning or misfire problems that foul the converter. If the engine is using oil, a new converter can be damaged the same way, so find that cause first.

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.