P0420: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 is set when the oxygen sensor behind the catalytic converter reports almost the same oxygen levels as the sensor in front of it. The engine computer expects the converter to buffer oxygen as it works; when that difference disappears, it concludes the converter on Bank 1 has fallen below its efficiency threshold.
Can I still drive with P0420?
Yes, in most cases
P0420 on its own is not a safety fault and the car will normally drive normally. It will not pass an emissions test. If you also hear rattling from under the car, lose power, or smell something burning, stop driving — a converter breaking up internally can block the exhaust.
What you will actually notice
- Check engine light on, usually with no change in how the car drives
- Failed emissions or inspection test
- Occasional sulphur or rotten-egg smell from the exhaust
- Slightly worse fuel economy over time
- In advanced cases, a rattle from under the car on start-up
Causes, ordered by how often they turn out to be the fault
1.Catalytic converter worn out with age and mileage
Very commonMost likely explanation past roughly 100,000 miles on an otherwise healthy engine.
2.Slow or lazy downstream oxygen sensor
CommonA sensor that responds sluggishly mimics a dead converter. Must be tested, not assumed.
3.Exhaust leak ahead of or between the sensors
CommonFresh air entering through a crack skews the readings and fakes the fault.
4.Aftermarket converter that does not meet OEM efficiency
CommonFrequent on cars where the converter was already replaced once with a budget part.
5.Engine running rich, or burning oil or coolant
Less commonThis is the reason a converter dies early. Replacing the converter without fixing it kills the new one too.
6.Damaged wiring or connector at the downstream sensor
RareWorth a look before condemning any part.
How to diagnose it, cheapest test first
Work down this list in order. The most expensive mistake with this code is replacing a costly part before ruling out a cheap one.
- 1
Read all stored codes and the freeze frame data
Free at most parts storesIf misfire or lean codes are also present, fix those first. They cause P0420 and will destroy a new converter.
- 2
Inspect the exhaust for leaks between the manifold and the rear sensor
Free to inspectListen for ticking on a cold start and look for soot streaks around joints and welds.
- 3
Compare upstream and downstream sensor waveforms on a scan tool
LowA healthy converter makes the rear sensor trace nearly flat while the front one oscillates.
- 4
Test converter temperature differential or exhaust back-pressure
Moderate, shop testConfirms whether the converter is actually dead before you spend on one.
- 5
Replace the downstream oxygen sensor if testing proved it lazy
ModerateOnly after testing. Blind replacement is the most common wasted repair on this code.
- 6
Replace the catalytic converter
HighLast resort, once the cheaper causes are ruled out.
What the repairs typically cost
| Repair | Estimated range |
|---|---|
| Downstream oxygen sensor replacementPart plus roughly an hour of labour. | $150 – $400 |
| Exhaust leak repairDepends entirely on whether it is a gasket, a weld or a section of pipe. | $100 – $500 |
| Catalytic converter replacementOEM parts sit at the top of this range; some makes reject aftermarket units. | $500 – $2,500 |
Estimate only. Ranges vary widely by region, vehicle and shop rate, and are not quotes. Always get a written estimate before authorising work.
Notes for specific makes
Toyota / Lexus
Frequently a genuine converter failure. Aftermarket units often re-trigger the code within weeks; OEM is usually the cheaper route overall.
Subaru
Very common at higher mileage. Check for head gasket seepage first — coolant entering the exhaust poisons converters.
Honda
Check the exhaust manifold for cracks, which are a known cause of a false P0420 on several engines.
Volkswagen / Audi
Often appears alongside lean codes. Resolve the lean condition before touching the converter.
Common questions
Will P0420 clear itself?
The light can go out if the monitor passes on a later drive cycle, but unless the underlying cause was fixed the code almost always returns within a few hundred miles.
Can I just replace the oxygen sensor and be done?
Only if a scan tool showed the sensor responding slowly. Replacing it on a hunch is the single most common wasted repair for this code.
Will P0420 damage my engine?
The code itself will not. A converter that is physically breaking apart can restrict the exhaust and cause noticeable power loss, so do not ignore rattling noises.
Can I pass an emissions test with P0420 stored?
No. A stored emissions code with the check engine light on is an automatic failure in essentially every inspection programme.
Is a cheap aftermarket converter a real fix?
Sometimes, but on several makes a non-OEM-spec unit does not reach the efficiency the computer expects and sets the code again.
Not sure which of these applies to your car?
Describe what the car is actually doing and get a diagnosis narrowed to your vehicle, or find a shop near you.
AI-generated and informational only. This is not a diagnosis of your specific vehicle and does not replace inspection by a qualified technician.