P0431

What it means

P0431 means the engine control unit has judged that the catalytic converter on bank 2, the side of the engine that does not contain cylinder 1, is not cleaning the exhaust well enough. It usually reaches this verdict by comparing the signal from the oxygen sensor before the catalyst with the one after it. A healthy catalyst smooths out the downstream signal, whereas a worn one lets it swing like the upstream signal.

Symptoms

• Engine warning light on
• Slight rise in fuel consumption
• Reduced power or sluggish acceleration in some cases
• Rotten-egg smell from the exhaust
• Failed emissions test at the MOT
• Other fault codes for misfires or fuelling stored alongside

Possible causes

• Worn or contaminated bank 2 catalytic converter through age, mileage or overheatinglikely
• Exhaust leak upstream of or at the bank 2 downstream oxygen sensor, letting in false airpossible
• Faulty or slow-responding bank 2 upstream or downstream oxygen sensorpossible
• Engine misfire or rich or lean running on bank 2 that has damaged the catalyst or distorted the readingspossible
• Oil or coolant being burned and contaminating the catalystpossible
• Damaged wiring or connector to the bank 2 oxygen sensorsrare
• Outdated engine control unit software where the manufacturer has issued a revised catalyst monitor calibration for that modelrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting engine load, temperature and fuel trims when the code set
2. Check for misfire, fuelling or oxygen sensor codes and deal with those first, as they can cause or mimic this fault
3. Visually inspect the exhaust from the manifold to the bank 2 catalyst for leaks, cracks, blown gaskets and damage
4. Inspect the bank 2 oxygen sensor wiring and connectors for chafing, heat damage, water or corrosion
5. Review short and long term fuel trims on bank 2 at idle and at about 2500 rpm to spot rich or lean running
6. Watch the live upstream and downstream sensor signals on bank 2; a downstream signal that mirrors the upstream one points to a spent catalyst
7. Check for oil or coolant consumption, and inspect the catalyst for rattling or heavy heat discolouration
8. Check whether a manufacturer software update or technical bulletin applies to the model and engine, as procedures vary

How to fix

• Replace the bank 2 catalytic converter once other causes have been ruled out, using a suitable approved part
• Repair exhaust leaks or replace faulty gaskets near the catalyst and sensors
• Replace any oxygen sensor proven faulty and repair damaged wiring or connectors
• Fix the underlying misfire, fuelling or oil or coolant consumption fault before or alongside any catalyst replacement
• Where a known software issue exists for that model, update the engine control unit calibration to the manufacturer's current release

Can I keep driving?

Nearby codes

• P0430GenericCatalyst System Efficiency Below Threshold Bank 2
• P0432GenericCatalyst 2 Efficiency Below Threshold Bank 2