P0422: Catalyst 2 Efficiency Below Threshold Bank 1

What does P0422 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.