P0422

What it means

P0422 is the generic SAE J2012 code for the main catalytic converter on Bank 1 operating below its efficiency threshold. The engine control unit compares the signals from the oxygen sensors before and after the catalyst, and the downstream signal is moving too much like the upstream one, which suggests the catalyst is no longer storing and processing oxygen properly. Where a car has more than one catalyst on a bank, the exact position of the 'main' catalyst depends on the model and engine layout, so check the manufacturer's layout.

Symptoms

• Engine management light on
• Usually no noticeable change in driving
• Slight rise in fuel consumption in some cases
• Failed emissions test at the exhaust gas reading
• Rotten egg (sulphur) smell from the exhaust
• Reduced power if the catalyst is partly blocked

Possible causes

• Worn or contaminated catalytic converter on Bank 1 (age, mileage, thermal damage)likely
• Engine misfires or rich or lean running that has overheated or fouled the catalystlikely
• Exhaust leak upstream of or near the downstream oxygen sensor, giving false readingspossible
• Faulty or slow downstream oxygen sensor, or damaged sensor wiring or connectorpossible
• Oil or coolant being burnt (worn rings, valve seals, head gasket) contaminating the catalystpossible
• Faulty upstream air-fuel ratio sensor causing poor fuel controlpossible
• Outdated engine control unit calibration where the manufacturer has issued a known update for this coderare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting load, engine temperature and fuel trim at the time the code set.
2. Look for and deal with any misfire, fuel trim or oxygen sensor codes first, as they can cause or mimic this fault.
3. Inspect the exhaust from manifold to catalyst for leaks, cracks, blown gaskets and damaged flexible joints, and listen for blowing with the engine running.
4. Check the oxygen sensor wiring and connectors for damage, heat melt, corrosion or contamination.
5. View live data for the upstream and downstream sensors at a steady warm idle and light load; a healthy downstream sensor should be fairly steady while the upstream one switches or changes rapidly.
6. Check short and long term fuel trims and look for signs of oil or coolant consumption, such as blue smoke or white deposits.
7. Check for any manufacturer service information or calibration updates relating to this code for the specific model and engine.
8. If the catalyst is suspected, check for restriction or compare temperatures before and after it, following the manufacturer's procedure.

How to fix

• Repair any exhaust leak and renew faulty gaskets or joints, then recheck the sensor readings.
• Repair the underlying fault that damaged the catalyst, such as misfires, rich or lean running, or oil or coolant burning, before fitting a new one.
• Renew a faulty downstream oxygen sensor or repair its wiring and connector.
• Renew the Bank 1 catalytic converter with a correct, approved part once it is confirmed as degraded.
• Apply a manufacturer calibration update only where an outdated calibration is a documented cause for the vehicle.

Can I keep driving?

Nearby codes

• P0421GenericCatalyst 1 Efficiency Below Threshold Bank 1
• P0423GenericHeated Catalyst Efficiency Below Threshold Bank 1