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Fault code P0420 means catalytic converter efficiency below threshold (bank 1). The comparison between the upstream and downstream lambda sensors indicates that the catalytic converter is no longer converting as it should.

Typical symptoms

• Engine light on
• Slightly increased consumption
• Acrid smell from the exhaust
• Often no change in driving

Most common causes

• Worn-out or damaged catalytic converter
• Lazy or faulty rear lambda sensor
• Exhaust leaks before or between the sensors
• Prolonged rich mixture due to injectors or MAF sensor
• Oil consumption poisoning the catalytic converter

How it is checked in diagnosis

• Real-time comparison of the two sensor signals
• Exhaust leak test
• Checking the fuel corrections
• Measuring back-pressure if clogging is suspected

Can you drive with code P0420?

Yes: in most cases driving stays normal, but it must be fixed for the MOT and to avoid getting worse.

How MasterTuning helps

We read the real values from the ECU, not just the code: that is the difference between identifying the component responsible and replacing parts by trial and error. After the repair we clear the fault and check that it does not come back. Centres throughout Italy.

Read more: vehicle database · find an installer · what is ECU remapping

Do you have code P0420 on your car? Book a diagnosis at your nearest MasterTuning centre.

Frequently asked questions

What does fault code P0420 mean?

Code P0420 means catalytic converter efficiency below threshold (bank 1). The comparison between the upstream and downstream lambda sensors indicates that the catalytic converter is no longer converting as it should.

Can you drive with fault P0420?

Yes: in most cases driving stays normal, but it must be fixed for the MOT and to avoid getting worse.

Is clearing code P0420 enough to fix it?

No: if the cause is still present the code comes back after a few kilometres. Clearing should only be done after the repair, with the diagnostic tool.

How much does it cost to fix code P0420?

It depends on the component responsible: the diagnosis is precisely what identifies it with real measurements, avoiding unnecessary replacements that cost far more than the correct repair.