The car compares the exhaust gas before and after the catalytic converter. When the two oxygen sensors start reporting similar readings, the converter has stopped doing its job of cleaning the exhaust, and the car stores this code. It is one of the most common codes on any petrol car past 150,000 km.
The car compares the exhaust gas before and after the catalytic converter. When the two oxygen sensors start reporting similar readings, the converter has stopped doing its job of cleaning the exhaust, and the car stores this code. It is one of the most common codes on any petrol car past 150,000 km.
The code is stored by the engine control unit the moment the reading falls outside the range the manufacturer allows. Stored is not the same as diagnosed: P0420 names the symptom the car detected, not the part that caused it. That is why the repair figures below sit in a range rather than at a single price.
In rough order of how often each one turns out to be the culprit:
Parts cost roughly the same wherever you are. What moves the total is the shop rate, which is why the same job is two to three times the price in the United States that it is in Italy. These figures assume an independent workshop, not a franchised dealer, and cover the cheapest and the most expensive of the causes listed above.
| Country | Parts | Labour | Total |
|---|---|---|---|
| United States | $50 to $970 | $60 to $360 | $110 to $1350 |
| United Kingdom | £40 to £800 | £40 to £240 | £80 to £1050 |
| Germany | €45 to €900 | €50 to €300 | €95 to €1200 |
| Italy | €45 to €900 | €20 to €140 | €70 to €1050 |
Based on 0.5 to 3.0 hours of book labour at local shop rates, plus parts at local retail. Indicative ranges, not quotes.
Usually yes, for a few days. The car will drive, but the fault will not clear on its own and the bill grows the longer it is left.
Yes, in every market we cover. The warning light alone fails the emissions check, and an emissions fault that is cleared on the morning of the test comes back as soon as the readiness monitors run again.
Clearing the code the day before a test is a false economy: erasing the memory also resets the readiness monitors, and a car that arrives with its monitors incomplete is refused on that ground alone in most testing regimes.
This is a negotiation code, not a dealbreaker. It is a real repair with a real bill, so the fair move is to price the fix and take it off the asking price. What matters more than the code is whether the seller knew about it.
The thing worth checking is not the code on its own. It is whether the memory was cleared recently. Every car keeps two numbers alongside the fault list: the distance travelled since the codes were last erased, and the state of the readiness monitors. A car with no stored faults, incomplete monitors and 30 km since the last reset was wiped in the seller's driveway, probably this week. That combination tells you more than any single code does.
TrustThisCar reads all of it during the viewing, in three minutes, with the kind of Bluetooth adapter that costs about 20 euro: stored, pending and permanent codes, readiness monitors, distance since the last reset, and a verdict in plain words you can send to the seller while you are still standing next to the car.
The car compares the exhaust gas before and after the catalytic converter. When the two oxygen sensors start reporting similar readings, the converter has stopped doing its job of cleaning the exhaust, and the car stores this code. It is one of the most common codes on any petrol car past 150,000 km.
Usually yes, for a few days. The car will drive, but the fault will not clear on its own and the bill grows the longer it is left.
Yes, in every market we cover. The warning light alone fails the emissions check, and an emissions fault that is cleared on the morning of the test comes back as soon as the readiness monitors run again.
It depends on the shop rate where you live: United States $110 to $1350, United Kingdom £80 to £1050, Germany €95 to €1200, Italy €70 to €1050. Parts are close to the same everywhere; labour is what moves the total.
Last reviewed 2026-10-03.