P0422
PowertrainMain Catalyst Efficiency Below Threshold (Bank 1)
The engine control module has detected that the main catalytic converter on Bank 1 (the side with cylinder #1) is operating below the expected efficiency threshold, typically indicated by similar readings from the upstream and downstream oxygen sensors.
ⓘ Information only. This page provides general educational information about fault code P0422. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0422 mean?
P0422 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Main Catalyst Efficiency Below Threshold (Bank 1).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
About Emissions codes
Emissions codes relate to systems that control or monitor exhaust gas composition, catalytic converters, oxygen sensors, exhaust gas recirculation (EGR), and the evaporative emissions (EVAP) system. These rarely affect drivability dramatically but can cause MOT failures.
P0422 is one of 258 codes in this category. Browse all Emissions codes →
What we know about P0422 specifically
We record the SAE J2012 standardised definition and the system this code belongs to, but we have not yet published detailed driver-reported symptoms, common causes, or step-by-step diagnostic guidance for P0422.
Rather than invent that information by generating generic content from a template, we leave it out. For specific guidance, the most reliable sources are:
- •your vehicle's own service manual or manufacturer-specific technical bulletins
- •a qualified mechanic with a manufacturer-level diagnostic tool
- •a marque specialist familiar with your make and model
Codes nearby
Codes adjacent to P0422 in the P-code series. Many fault codes appear together with their neighbours because they relate to the same circuit or component.
Other Emissions codes
Other fault codes in the same system as P0422. The ones with detailed guides have plain-English explanations, symptoms, causes, and diagnostic steps.
Spotted an issue or have something to add? Mechanics, technicians, and drivers who have first-hand experience with P0422 can submit a correction or expansion. We review every submission before publishing, mechanic submissions are prioritised.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →