P0423

What it means

P0423 means the engine control unit has judged that the main (heated) catalytic converter on bank 1 is not storing and converting exhaust gases as well as it should. It reaches this view by comparing the signal from the oxygen sensor after the catalyst with the one before it during a monitoring test. If the downstream signal switches too much like the upstream one, the catalyst is rated below its efficiency threshold.

Symptoms

• Engine warning light on
• Failed emissions or MOT exhaust test
• Rotten egg (sulphur) smell from the exhaust
• Slight loss of power or poor fuel economy
• Rattling from the exhaust if the catalyst substrate has broken up
• Often no noticeable driveability change at all

Possible causes

• Catalytic converter on bank 1 worn out, contaminated or damagedlikely
• Exhaust leak before or near the downstream oxygen sensor letting in extra airpossible
• Faulty or slow upstream or downstream oxygen sensor on bank 1possible
• Engine misfire or rich or lean running that has overheated or fouled the catalystpossible
• Oil or coolant burning in the engine contaminating the catalystpossible
• Damaged wiring or connectors to the bank 1 oxygen sensorsrare
• Outdated engine control software where the manufacturer has issued a revision for the catalyst monitor (model dependent)rare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting any misfire, fuel trim or oxygen sensor codes that should be dealt with first
2. Check for exhaust leaks from the manifold to the downstream sensor, looking for soot marks and listening on a cold start
3. Inspect the bank 1 oxygen sensor wiring and connectors for damage, heat melt or contamination
4. Review long and short term fuel trims at idle and at 2500 rpm to spot rich or lean running
5. Check for misfire counts, and for signs of oil or coolant consumption such as blue smoke or unexplained coolant loss
6. Watch live upstream and downstream oxygen sensor readings at operating temperature; a healthy catalyst gives a fairly steady downstream signal while the upstream one switches rapidly
7. Check for restriction or overheating of the catalyst, for example with a temperature comparison before and after it or a backpressure test, following the procedure for the specific engine
8. Check whether a software update or technical bulletin applies to the vehicle

How to fix

• Repair any exhaust leak found ahead of or near the downstream oxygen sensor
• Replace a faulty oxygen sensor that is confirmed slow, biased or failed
• Cure the underlying misfire, fuelling or oil or coolant consumption fault before fitting a new catalyst
• Replace the bank 1 catalytic converter with a correct specification unit once the cause of failure has been fixed
• Repair damaged wiring or connectors to the oxygen sensors
• Apply a manufacturer software update only if one is documented for this fault on the vehicle

Can I keep driving?

Nearby codes

• P0422GenericCatalyst 2 Efficiency Below Threshold Bank 1
• P0424GenericHeated Catalyst Temperature Below Threshold Bank 1