Symptoms of P0420
- Check-engine light, usually with no change in how the car drives.
- A failed emissions test.
- Sometimes a faint sulphur / rotten-egg smell.
- Rarely, a rattle from inside the converter.
Common causes of P0420
- A worn or failed catalytic converter on bank 1.
- A lazy or failed downstream oxygen sensor.
- An exhaust leak upstream of the converter.
- An underlying misfire or fuel-trim fault that damaged the cat.
Useful checks before replacing parts
- Inspect for exhaust leaks, sensor wiring faults, and incorrect oxygen or air-fuel sensor readings before condemning the catalyst.
- Review fuel trims, misfire history, oil consumption, and coolant contamination because upstream problems can damage or mislead catalyst monitoring.
- Confirm that the catalyst monitor has run under valid conditions after any repair.
How serious is P0420, and can you keep driving?
- Urgency
- Low to moderate for driving, certain to fail a test
- Safe to drive?
- Generally yes — but the car will not pass an emissions test.
Evaporative-emission and similar codes mostly describe a system that stops fuel vapour escaping, so the car drives normally with one set. The practical cost is the test: a stored code, or monitors that have not completed, is a failure on the spot in most jurisdictions. Catalyst-efficiency codes deserve more care, because they frequently report a converter that has been damaged by something upstream — a misfire or a rich mixture — which is still there and still doing it.
Clearing P0420, and making it stay gone
Clearing P0420 with a scan tool switches the lamp off; it does not fix anything, and it throws away the freeze-frame snapshot the car saved when the fault set. That snapshot is the most useful diagnostic evidence you have, because it records the load, RPM and temperature at the moment the test failed. Read it before you erase it.
Left alone, the lamp goes out on its own once the monitor that failed runs and passes on three consecutive driving cycles. The code stays in history after that, which is how a workshop can still see it. Disconnecting the battery is not a shortcut — it clears the stored codes and every learned fuel-trim value with them, so the car relearns from scratch and the underlying fault simply sets the code again.
On vehicles from roughly 2010 onward there is also a permanent DTC, which no scan tool can erase at all. It clears only when the monitor genuinely passes. That is deliberate: it stops a code being wiped in the car park outside an emissions test.
What P0420 costs to put right
There is no honest single figure for P0420, and any page that gives you one is guessing. The code names a circuit or a failed monitor, not a part — the same code can be a chafed wire, a connector full of water, or the component itself, and those are three very different bills.
The number worth budgeting for is diagnostic time. An hour of it is almost always cheaper than the second part you buy after the first one does not fix it, and parts fitted on a guess are rarely refundable once installed. If you are getting a quote, ask what the diagnosis found rather than what is being replaced — a emissions fault traced to a specific cause is a repair, and a list of parts is a hopeful shopping list.
Questions people ask about P0420
What does P0420 mean?
P0420 points to catalyst efficiency below threshold on bank 1. The safe reading is that sensor data suggests the catalyst is not storing oxygen as expected under the test conditions.
Can I drive with P0420?
Generally yes — but the car will not pass an emissions test. Evaporative-emission and similar codes mostly describe a system that stops fuel vapour escaping, so the car drives normally with one set. The practical cost is the test: a stored code, or monitors that have not completed, is a failure on the spot in most jurisdictions. Catalyst-efficiency codes deserve more care, because they frequently report a converter that has been damaged by something upstream — a misfire or a rich mixture — which is still there and still doing it.
What causes P0420?
The usual candidates are a worn or failed catalytic converter on bank 1; a lazy or failed downstream oxygen sensor; an exhaust leak upstream of the converter; an underlying misfire or fuel-trim fault that damaged the cat. The code tells you which test failed, not which of those did it — that is what the checks on this page are for.
Will P0420 clear itself?
Left alone, the lamp goes out on its own once the monitor that failed runs and passes on three consecutive driving cycles. The code stays in history after that, which is how a workshop can still see it. Disconnecting the battery is not a shortcut — it clears the stored codes and every learned fuel-trim value with them, so the car relearns from scratch and the underlying fault simply sets the code again.
Does P0420 mean I need to replace a part?
Do not replace the catalytic converter based only on the stored code. Verify the monitor data and the conditions that caused the code.
How much does it cost to fix P0420?
There is no honest single figure for P0420, and any page that gives you one is guessing. The code names a circuit or a failed monitor, not a part — the same code can be a chafed wire, a connector full of water, or the component itself, and those are three very different bills.
Related OBD-II codes
These nearby emissions entries can help compare wording and pin down the fault area.