The P0420 code means "catalyst system efficiency below threshold (bank 1)". The engine control unit has compared the oxygen sensor signals before and after the catalytic converter and concluded that the cat is not cleaning the exhaust gases as it should. It is a generic OBD2 code, the same on every make.
How the control unit detects it
The sensor upstream of the cat switches constantly because the engine keeps adjusting the mixture. A healthy catalytic converter stores oxygen and smooths out that signal, so the downstream sensor should show a fairly steady reading. When the downstream signal starts to look like the upstream one, the control unit decides the cat has lost its capacity and logs P0420.
"Bank 1" is the side of the engine with cylinder 1. Inline four-cylinder engines have only one bank; if your engine is a V and you see P0430, it is the same diagnosis on bank 2.
Most common causes
| Cause | Diagnostic clue |
|---|---|
| Worn or damaged catalytic converter | Downstream sensor switching almost like the upstream one, no other faults |
| Faulty or slow downstream oxygen sensor | Flat or out-of-range signal, or the sensor's own fault codes |
| Exhaust leak before or near the sensors | Blowing exhaust, soot at joints, lean readings |
| Earlier misfires | P0300 to P030X codes in the history |
| Mixture too rich or too lean | High fuel trims, P0171/P0172 codes |
| Burning oil or coolant | Bluish or white smoke, level dropping with no visible leaks |
Don't replace the cat blindly. If the cause is a misfire or a wrong mixture, a new catalytic converter will fail again. Rule out the rest of the list first.
How to diagnose it step by step
- Read every engine fault, not just P0420. If there are misfire, sensor or mixture faults, start with those.
- Save the freeze frame so you know the conditions when it appeared (temperature, load, speed).
- Warm the engine up to operating temperature. The cat and the sensors need heat to give reliable readings.
- Open live data and display the bank 1 upstream and downstream sensors together. Compare how they behave at idle and while holding steady revs.
- Check the short-term and long-term fuel trims. Values that are well off point to an intake leak, an injection problem or the air flow meter.
- Use the O2 sensor test (mode 05) and the on-board monitor test (mode 06), if the car offers them, to see the results of the control unit's self-tests.
- Inspect the exhaust when cold, looking for leaks, loose joints or knocks to the cat.
- Repair, clear the code and drive for several days. Check again with the tool that the catalyst monitor shows as complete in I/M Readiness.
Safety: the catalytic converter and exhaust get extremely hot. Inspect them with the engine cold, with the car securely on axle stands or a lift, and never get under a car held up only by a jack.
Symptoms that may come with it
In many cases the only symptom is the warning light. When the cat is badly deteriorated or partly blocked, others can appear: a rotten egg smell from the exhaust, loss of power as the revs rise, a metallic rattle from loose material inside the cat when you blip the throttle, or slightly higher fuel consumption. If you notice a clear lack of power, suspect a blocked cat rather than just low efficiency.
What usually fixes it
- Faulty downstream sensor: replace the sensor and clear the code. It is one of the cheapest repairs, which is why it is worth ruling out early.
- Exhaust leak: replace the affected gasket or clamp.
- Misfires or mixture problems: fix the cause first, then check whether P0420 comes back.
- Worn-out cat: replace it with a type-approved unit for your vehicle, once everything above has been ruled out.
Can you drive with a P0420?
In the short term the car usually runs normally, but it emits more than it should, and a lit warning light can be a reason to fail the roadworthiness test (NCT, MOT or your country's equivalent). And if the real cause is something else (misfires, for example), carrying on driving can make it worse. Get it diagnosed as soon as you can.
Which iCarsoft tools help with a P0420
For this diagnosis you need more than read and clear: live sensor data and the OBD2 test modes. These are the tools that cover it:
- CR Pro: all ten OBD2 modes, including live data, freeze frame, O2 sensor test and on-board monitors, plus diagnostics on every system.
- CR Pro+: adds bidirectional control and service functions such as O2 sensor and misfire checks.
- CR Max and CR Max P: 7" and 10" tablets with 50 functions; the big screen makes it easier to compare both sensors in real time.
If you are still choosing, what a diagnostic tool can do explains what you need for your kind of use, and every model is under multi-brand diagnostic tools. Once it is repaired, follow our guide to clearing the engine warning light.
