Can I drive with code P0420?

Usually yes, if the light stays on steadily and the engine runs normally. The fault does not damage the engine by itself unless the inside of the converter breaks up and blocks the exhaust, but it does raise emissions. If the light flashes or the engine runs badly, stop driving and have the car checked.

Will my car pass its MOT with code P0420?

In the UK, an engine management light that shows a fault is a major defect, so the car fails its MOT. This applies to most petrol cars first used from 1 July 2003. Elsewhere in Europe it depends on your country’s inspection rules; the EU roadworthiness directive lists an OBD read-out showing a significant malfunction as a reason for failure on petrol cars. This fault also affects exhaust emissions, so the car may fail the emissions test as well. It is safer to have it fixed before the test.

Is replacing the oxygen sensor enough?

Only if the diagnosis shows the sensor is at fault. The most common cause is the catalytic converter, so replacing the sensor without testing often does not clear the fault. A mechanic first compares the signals from both sensors and checks the exhaust for leaks.

What does code P042000 mean?

It is the same fault as shown by the diagnostic software for Volkswagen, Audi, Škoda and SEAT: P0420 with two extra zeros at the end. It means the same as P0420.

P0420 Code: Meaning, Causes & Can You Drive?

When the code is set

The ECM compares the signals from the oxygen sensors before and after the catalytic converter. A working converter stores oxygen, so the rear sensor signal changes very little. When the rear sensor starts to follow the front one, the ECM concludes that the converter is no longer doing its job and stores the code. Bank 1 is the side of the engine with cylinder 1; inline engines have only one bank, so on them P0420 covers the whole exhaust.

Can I keep driving?

Usually safe to drive

If the light stays on steadily and the engine runs as usual, you can normally keep driving and mention the code at your next service. If the light flashes, the engine runs unevenly or something rattles under the car, stop when it is safe and speak to a mechanic.

Symptoms

The check engine light is on, often with no other symptoms.
A rotten-egg (sulphur) smell from the exhaust.
Rattling under the car if the inside of the converter has broken up.
Less power and poorer throttle response if the converter is blocked.
High readings in an exhaust emissions test.

Common causes

A worn or damaged catalytic converter – the most common cause.
A faulty or ageing oxygen sensor before or after the converter, or its wiring.
An exhaust leak ahead of the rear oxygen sensor.
Outdated engine control module software (on some models).
Misfires, leaking injectors, or burning oil or coolant, all of which damage the converter.

How to find the cause

A mechanic usually works like this:

Read all stored codes and the freeze frame data the ECM recorded when the fault appeared.
Fix any misfires and exhaust leaks first, because they can set the code on their own or ruin a new converter.
Compare the front and rear oxygen sensor signals on a test drive.
Check that the converter outlet runs hotter than the inlet, and inspect it for damage.
After the repair, clear the code and confirm on a test drive that the ECM passes its catalyst test again.

Notes by make

Audi: In the US, Audi issued a service bulletin for codes P042000 and P043000 on some models with the 3.0 TFSI engine (2011–2015 S4 and S5, 2012–2014 A6, A7 and A8, and 2013–2015 Q5): the cause was the engine control module software and the fix was a software update. A workshop can check whether this applies to your car.

Other ways the code is written

Scan tools and workshop printouts sometimes show the code with extra characters:

P042000

Related codes

Frequently asked questions

Evoca fault code diagnostics is an informational aid and does not replace a professional inspection of your vehicle. Only an inspection of the vehicle at a workshop can confirm the cause of a fault.

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P0420 code: catalytic converter efficiency below threshold