What are the common causes of P0420?

Failing catalytic converter. Oxygen sensor failure (upstream or downstream). Exhaust leak before the O2 sensor. Engine misfire dumping raw fuel into the converter. Oil or coolant burning from engine issues.

Test upstream and downstream oxygen sensor switching. Check for exhaust leaks with a smoke test. Monitor fuel trim values using an OBD-II scanner. Diagnose and fix misfires before condemning the converter. Replace the catalytic converter if efficiency remains below threshold after fixing other issues.

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

System: Auxiliary emission controls (P code)
Lamp: Check Engine Light
Category: Powertrain - Auxiliary Emissions Controls

What it means

P0420 means Catalyst System Efficiency Below Threshold (Bank 1).

Check these first

Failing catalytic converter
Oxygen sensor failure (upstream or downstream)
Exhaust leak before the O2 sensor
Engine misfire dumping raw fuel into the converter
Oil or coolant burning from engine issues

What to do next

Test upstream and downstream oxygen sensor switching
Check for exhaust leaks with a smoke test
Monitor fuel trim values using an OBD-II scanner
Diagnose and fix misfires before condemning the converter
Replace the catalytic converter if efficiency remains below threshold after fixing other issues

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Updated

Generic SAE J2012 definition. Last updated 2026-08-30.