P0422: Catalyst 2 Efficiency Below Threshold Bank 1

Catalyst 2 is the second catalyst in the flow direction of bank 1 (the side with cylinder 1) – the main or underfloor catalyst behind the close-coupled one; the older wording was Main Catalyst. P0422 means it no longer converts enough. Only GM's small diesels are documented with this code: they assess the underfloor oxidation catalyst separately from the close-coupled one (P0421) by heat balance during filter regeneration, with an exhaust fuel injector ahead of it. Petrol engines with several catalysts usually report P0420 instead.

Medium severity. Can I keep driving?You can usually keep driving, unless you notice a loss of power, which may point to a blockage. The car may not pass an emissions inspection.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

Check engine light on
Emission test problems
Diesel: possibly more frequent filter regenerations

Most likely causes

Main or underfloor catalyst aged or poisoned
Diesel: exhaust fuel injector coked, doses too little
Temperature or oxygen sensor after the catalyst wrong
Misfire or mixture fault as prior damage

What to check, in order

Start with the simplest. You can tick off each step.

First resolve any misfire or fuel mixture codes
Look for exhaust leaks between the engine and the second sensor
Use live data to compare the signals from both oxygen sensors
Check for a blockage by measuring exhaust back pressure
Replace the catalytic converter only at the end of the diagnosis

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0422 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
22: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0422?

You can usually keep driving, unless you notice a loss of power, which may point to a blockage. The car may not pass an emissions inspection.

What causes code P0422?

The most common causes are: main or underfloor catalyst aged or poisoned; diesel: exhaust fuel injector coked, doses too little; temperature or oxygen sensor after the catalyst wrong.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.