P0420

What it means

P0420 on an Audi means the engine control unit has judged that the Bank 1 catalytic converter is not cleaning the exhaust as well as it should. It compares the signal from the sensor before the catalyst with the one after it, and if the rear sensor is switching too much like the front one, the catalyst is storing and converting too little oxygen. The cause can be the catalyst itself, but misfires, exhaust leaks and faulty oxygen sensors can set the same code.

Symptoms

• Engine management light on
• Little or no change in driveability in many cases
• Slightly higher fuel consumption
• Rough running or hesitation if a misfire is involved
• Sulphur or rotten-egg smell from the exhaust
• Failed emissions test

Possible causes

• Catalytic converter worn, contaminated or damaged so its oxygen storage capacity has droppedlikely
• Engine misfires or an overly rich or lean mixture that has overheated or contaminated the catalystlikely
• Exhaust leak ahead of or near the catalyst or the rear oxygen sensor, letting in false airpossible
• Ageing or slow-responding front or rear oxygen sensor (air/fuel sensor), or damaged sensor wiring or connectorpossible
• Fuel trim problems from an air leak, fuel delivery fault or leaking injectorpossible
• Oil or coolant burning in the engine contaminating the catalyst; the cause depends on the engine fittedpossible
• Outdated engine control unit software where the manufacturer has issued a revised calibration for the model and enginerare

How to diagnose

1. Read all stored and pending codes and the freeze frame data; look for misfire, fuel trim or oxygen sensor codes that should be fixed first
2. Check for service bulletins or software updates relevant to the model, engine and year
3. Inspect the exhaust from the manifold to the rear sensor for leaks, cracks, loose joints and damaged flexible sections
4. Inspect the oxygen sensor wiring and connectors on Bank 1 for damage, heat marks, oil or water contamination
5. Check short and long term fuel trims at idle and at about 2500 rpm for signs of an air leak or fuel fault
6. Check live misfire counters and inspect ignition components and injectors if any misfire is recorded
7. With the engine fully warm, compare front and rear sensor live data; the rear signal should be steadier than the front
8. Check for oil or coolant consumption and signs of contamination before condemning the catalyst

How to fix

• Repair any exhaust leak found ahead of or around the catalyst and sensors
• Fix the root cause of misfires, such as faulty ignition coils, spark plugs or injectors, before anything else
• Replace a faulty oxygen sensor or repair damaged wiring and connectors
• Correct the cause of abnormal fuel trims, such as an intake air leak or fuel delivery fault
• Replace the catalytic converter with a suitable one for the model and engine once other causes are ruled out and the cause of its failure is fixed
• Apply an updated engine control unit calibration only where the manufacturer lists it as a fix for this code on that model

Can I keep driving?

This code for other makes

• P0420GenericCatalyst System Efficiency Below Threshold Bank 1

Nearby codes

• P0401AudiRestriction in the EGR passages usually caused by carbon buildup, The EGR Valve is defective, Lack of proper vacuum or electrical signal to the EGR valve, Malfunctioning EGR Vacuum supply solenoid, Lack of proper EGR system feedback to the computer from the: Manifold Absolute Pressure Sensor (MAP), Differential EGR Pressure Feedback Sensor (DPFE), EGR Valve Position Sensor (EVP), EGR Temperature Sensor
• P0430AudiDefective Catalytic Converter, Internal engine damage resulting in high oil consumption and or a leaking Head Gasket has damaged the Catalytic Converter