MasterTuning Fault code dictionary (DTC)

P0420 — Catalyst efficiency below threshold

Severity: Warning Can you drive? Yes, no known limitation Make: All makes (OBD-II standard)
MeaningThe two lambda sensors, before and after the catalyst, read values that are too similar: the catalyst is no longer doing its job.
FamilyPowertrain (P)

Typical symptoms

Engine warning light on
No driving symptoms in most cases
Problems at the roadworthiness test

Common causes

Catalyst worn out by mileage
Aged rear lambda sensor
Exhaust leaks upstream of the catalyst
Oil consumption or rich mixture that ruined the catalyst

What to do

Before replacing the catalyst — an expensive part — rule out the rear sensor and exhaust leaks, which give the same code. And fix the cause: a new catalyst on an oil-burning engine will not last.

Frequently asked questions

What does code P0420 mean?

Catalyst efficiency below threshold. The two lambda sensors, before and after the catalyst, read values that are too similar: the catalyst is no longer doing its job.

Can I keep driving with code P0420 active?

Yes, no known limitation

Common causes — P0420

Catalyst worn out by mileage Aged rear lambda sensor Exhaust leaks upstream of the catalyst Oil consumption or rich mixture that ruined the catalyst

How do you fix code P0420?

Before replacing the catalyst — an expensive part — rule out the rear sensor and exhaust leaks, which give the same code. And fix the cause: a new catalyst on an oil-burning engine will not last.

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