Code P0420 means the catalytic converter is not converting exhaust gases as it should: comparing the upstream and downstream lambda sensors shows it is working poorly. It does not always mean the converter has to be replaced.
What code P0420 means
Catalytic converter efficiency below threshold (bank 1).
Typical symptoms
- Check engine light on
- Slightly higher fuel consumption
- Acrid or rotten-egg smell from the exhaust
- Often no noticeable change in driving
- Loss of power if it is clogged
Most likely causes
| Cause | How common | What to do |
|---|---|---|
| Rear lambda sensor slow or faulty | Very common | Check the signal and replace (much cheaper) |
| Exhaust leak before or between the sensors | Common | Leak test and repair |
| Worn-out catalytic converter | Common | Replace with an approved part |
| Prolonged rich mixture (injectors, MAF) | Common | Fix the cause, then re-check the converter |
| Oil consumption poisoning the converter | Less common | Fix the consumption, then replace |
How it is diagnosed
- Compare both lambda sensor signals in real time.
- Exhaust leak test.
- Read fuel trims.
- Measure back-pressure if blockage is suspected.
Can you drive with code P0420?
Yes, in most cases driving stays normal. It has to be fixed for the inspection and to avoid making it worse: a converter under stress can clog and lose power.
How much does it cost to fix?
| Work | Indicative cost |
|---|---|
| Rear lambda sensor | €100-250 |
| Exhaust leak repair | €50-300 |
| Approved aftermarket catalytic converter | €200-600 |
| Original catalytic converter | €600-1,500 and over |
Figures are indicative market ranges, useful for orientation: the real cost depends on model, engine, parts chosen and labour hours. For an exact quote we need the vehicle registration number.
How MasterTuning helps
We read the real values of the control unit, not just the code: that is the difference between finding the faulty component and replacing parts by trial and error. After the repair we clear the error and check that it does not come back. Find the nearest centre or contact us.
Got code P0420 on your car? Book a diagnosis at the nearest MasterTuning centre.
Frequently asked questions
Does P0420 mean I need a new catalytic converter?
Not necessarily: it can be the rear lambda sensor, an exhaust leak or a rich mixture. These must be ruled out first.
Can I pass the inspection with P0420?
Not with the light on and the code stored: it has to be fixed and emissions must be within limits.
How much does it cost to fix P0420?
From €100-250 if it is the lambda sensor, €200-600 for an approved aftermarket converter; the original can exceed €1,500.
Does P0420 go away by itself?
Sometimes the light goes off after a few cycles, but if the cause is still there the code returns.
