Question 1

What does P0420 mean?

Accepted Answer

P0420 - Catalyst System Efficiency Below Threshold (Bank 1). Your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream O2 sensor is showing a pattern too similar to the upstream sensor. This doesn't necessarily mean the cat is dead — the O2 sensors might be the real problem.

Question 2

What are the symptoms of P0420?

Accepted Answer

Check engine light, May still pass emissions if borderline, Possible sulfur/rotten egg smell, Usually no drivability issues

Question 3

What causes P0420?

Accepted Answer

Degraded catalytic converter (age/mileage), Faulty downstream O2 sensor giving false readings, Exhaust leak before downstream O2 sensor, Engine running rich, Incorrect fuel, Oil burning contaminating catalyst, Engine misfires damaging catalyst

Question 4

How much does it cost to fix P0420?

Accepted Answer

Typical repair cost: $100-$2,500. Before replacing the catalytic converter ($500-2,500), check the downstream O2 sensor first ($100-200). A lazy sensor is the most common false trigger. Also check for exhaust leaks between cat and rear O2 sensor.

P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

Don't guess — diagnose P0420 the right way

Common symptoms of P0420

Common causes of P0420

Repair cost & difficulty

Vehicles commonly affected

Related trouble codes