P0421

Catalyst 1 Efficiency Below Threshold Bank 1

Emissions (catalyst) Generic OBD-II Medium

What it means

P0421 means the warm-up catalytic converter on bank 1 isn't cleaning up exhaust gases as efficiently as the ECM expects, based on the upstream and downstream oxygen sensor readings. It's often an aged converter, but a faulty O2 sensor, an exhaust leak, or prior misfire/fuel-mixture problems can trigger it too. The engine drives normally — the practical impact is failing emissions and the warning light.

Medium

Can I keep driving? Safe to drive — this is an emissions-efficiency fault, not a running problem. The engine drives normally, but the light stays on and it will fail an emissions test.

Real-world scan data

Seen in

862

scans in the past year

Found in

0.1%

of scans

0.2%

of scans that found faults

Appears alongside · share of P0421 scans

P244Bin 67.6% of scans

P0850in 66.0% of scans

P0704in 65.9% 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
Fails an emissions test
Occasional sulphur/rotten-egg smell from the exhaust
Little to no change in how the engine drives

Common causes

Most likely first

Aged or failing warm-up catalytic converter on bank 1Likely
Faulty downstream oxygen sensor giving a false efficiency readingCommon
Exhaust leak between the converter and sensorsCommon
Upstream misfire or rich/lean running that has degraded the catalystLess often

How to diagnose & fix

Work through it in order

Rule out and repair any active misfire or fuel-trim codes first — they damage catalysts.
Compare upstream and downstream O2 sensor traces; near-identical switching points to a worn catalyst.
Inspect the exhaust for leaks near the converter and sensors.
Replace the warm-up catalytic converter if it's genuinely below efficiency and the sensors are good. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0421?+

Yes. The engine runs normally; the concern is emissions — the car will fail a smog test and may smell of sulphur until the catalyst is sorted.

Can I just clear P0421?+

It'll return after the ECM re-runs its catalyst monitor unless the underlying converter, sensor, or engine cause is fixed.

Read & clear P0421 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX