fault code guide
P0430: Catalyst System Efficiency Below Threshold (Bank 2)
Your engine computer has detected that the catalytic converter on bank two is not cleaning exhaust gases effectively. You can usually continue driving for now, but your engine produces higher emissions and may use more fuel.
How it shows up: The amber engine management light will stay illuminated on your dashboard, and you may notice a slight drop in fuel economy or a sulphur smell from the exhaust. The specific P0430 code is confirmed by plugging an OBD scanner into the diagnostic port.
Updated 9 November 2026
Can I keep driving?
Service soon
The car remains drivable, but running with an inefficient catalyst increases tailpipe pollution and will eventually cause an MOT failure or further exhaust damage.
What to do right now
- 1Check your instrument cluster to confirm the engine management light is glowing steadily rather than flashing.
- 2If the light flashes or the engine stumbles heavily, pull over safely and switch off the engine to prevent melting the converter.
- 3Book an inspection with a reputable garage or motor technician within the next couple of weeks.
Likely causes
Most likely first.
- Likely
Worn or degraded catalytic converter
Over time, the precious metal washcoat inside the bank two converter breaks down, meaning it can no longer store oxygen and convert harmful exhaust gases.
- Possible
Exhaust leak upstream or near the converter
A cracked exhaust manifold, blown gasket, or hole in the pipe lets outside air in, which fools the downstream oxygen sensor into reading poor converter performance.
- Possible
Faulty downstream oxygen sensor
The oxygen sensor fitted behind the catalytic converter monitors its efficiency, and an aged or contaminated sensor can report incorrect voltage data.
- Rare
Unburnt fuel from prior engine misfires
Spark plugs or ignition coils failing upstream allow raw fuel to reach the hot exhaust, which overheats and physically melts the ceramic honeycomb core.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Check engine light status
Look at the dashboard while driving. A steady amber engine light means you can safely drive home, but a flashing light means raw fuel is entering the exhaust and you must stop right away.
- 2
Listen for exhaust blowing noises
Park the car safely, apply the handbrake, and start the engine from cold. Listen near the engine bay and under the car for ticking, hissing, or blowing sounds that indicate an exhaust gas leak.
- 3
Inspect wiring to the bank two sensors
Once the exhaust has cooled completely, look underneath bank two (the side without cylinder one) to check if any sensor wiring harnesses are melted against hot pipes or physically damaged.
- 4
Scan for companion fault codes
Plug in an inexpensive code reader to see if misfire codes (such as P0300) or fuel trim codes are present, as unaddressed engine running faults will destroy a replacement converter.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total bill depends heavily on whether your car uses an aftermarket bolt-on converter or an expensive manifold-integrated unit from the main dealer.
Bank two catalytic converter replacement
- Do it yourself
- €280–950£240–820 in the UK
- At a garage
- €450–1,400£380–1,200 in the UK
Downstream oxygen sensor replacement
- Do it yourself
- €60–160£50–140 in the UK
- At a garage
- €140–280£120–240 in the UK
Exhaust manifold gasket or leak repair
- Do it yourself
- Garage job
- At a garage
- €120–350£100–300 in the UK
When to go to a garage
- The engine management light starts flashing or the engine loses power and shudders.
- You smell raw petrol, heavy sulphur, or exhaust fumes entering the vehicle cabin.
- A replacement sensor does not clear the code and professional exhaust back-pressure or emissions testing is required.
Questions drivers ask
Can I keep driving my car with code P0430?
Yes, you can usually continue normal driving in the short term provided the engine runs smoothly and the light remains steady. However, prolonged driving with an underlying fuel mixture problem or broken core can cause a severe exhaust blockage, eventually leaving you stranded.
What is the difference between P0420 and P0430?
Both fault codes describe the exact same catalytic converter efficiency failure. The difference is location: P0420 points to bank one (the side containing cylinder one), whereas P0430 points to bank two on V6, V8, or boxer engines.
Will code P0430 cause an MOT or NCT failure?
In the UK, a lit engine management light is an immediate MOT failure on petrol cars registered from July 2003 and diesel cars from July 2008. In Ireland, the engine light alone results in an Advisory Pass on the NCT, but your car will still fail if tailpipe emissions exceed legal limits.
Will clearing the fault code with an OBD scanner fix the problem?
Clearing the code will turn the warning light off momentarily, but the engine control module will run its emissions self-tests as you drive. Once the vehicle completes its drive cycle, the computer will detect the poor catalyst efficiency again and bring the light back on.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.