P0422 Main Catalyst Efficiency Below Threshold (Bank 1)

P0422 indicates main catalyst efficiency below threshold on Bank 1. The controller believes the Bank 1 catalyst is not storing oxygen or cleaning exhaust as effectively as expected.

Code details

Generic OBD-II · P0

System: Emissions
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the Bank 1 catalyst until oxygen sensors, exhaust leaks, fuel control, and misfire history have been checked.

Symptoms of P0422

Check-engine light, often with no obvious change in how the car drives.
A failed emissions test or incomplete readiness monitors.
Sometimes a faint fuel-vapour smell (on EVAP codes).

Common causes of P0422

A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
Exhaust leaks, fuel-trim problems, or a fault in a related monitor.

Useful checks before replacing parts

Compare upstream and downstream oxygen sensor activity, fuel trims, catalyst monitor data, exhaust leaks, freeze-frame load, and catalyst temperature behavior.
Inspect for misfires, rich or lean operation, oil or coolant contamination, exhaust leaks near sensors, damaged substrate, and incorrect sensor placement.
Confirm the engine is running correctly before judging catalyst efficiency because fueling and misfire faults can mimic a weak catalyst.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.