What does code P0420 mean?

P0420: Catalyst System Efficiency Below Threshold

Severity: low

What it means: The bank 1 catalytic converter isn't cleaning the exhaust as well as it should, judged by comparing the O2 sensors before and after it.

Safe to drive? Usually safe to drive in the short term, but it will fail an emissions test. If the car has power loss or a rotten-egg smell, or you also have misfire codes, have it checked right away.

Common symptoms

Check-engine light
Often no drivability change
Failed emissions test
Sometimes a sulfur smell or sluggishness

Likely causes, ranked

Worn-out catalytic converter$1,348Shop
CarMD 2024 national average about $1,348
Faulty or slow downstream/upstream O2 sensor$254DIY
CarMD 2024 national average about $254
Exhaust leak near the O2 sensorsShop
Misfires or a rich mixture that damaged the converterShop

Vehicle-specific notes

2003–2019 Toyota Camry: Usually an aging catalytic converter. Toyota technicians also check upstream air/fuel and O2 sensor behavior and fuel trims before replacing the cat. Read the full answer · Source

What it means: The bank 1 catalytic converter isn't cleaning the exhaust as well as it should, judged by comparing the O2 sensors before and after it.

Severity: Low. Safe to drive? Usually safe to drive in the short term, but it will fail an emissions test. If the car has power loss or a rotten-egg smell, or you also have misfire codes, have it checked right away.

DIY or shop?

Fix any misfire or fuel codes first. O2 sensors can be a DIY job, but get a shop diagnosis before buying a converter.

Before you replace parts

Read all stored codes and freeze-frame data. Fix misfire, fuel or sensor codes before chasing emissions codes.
Check the simple things first: connectors, vacuum hoses, fluid levels and the gas cap.
Clear the code only after a repair, then confirm it stays off after a few drives.
Check your VIN for recalls and service bulletins.

Safety / mechanic note

Generally not urgent, but a code only points to a system, not always a specific part. Verify with a mechanic before buying expensive parts.