Catalytic converter missing or defective

What causes it

• Theft, which remains common on hybrids and on cars with good ground clearance, and leaves a cleanly cut pipe behind
• Deliberate removal, often as part of a performance modification or to avoid replacing an expensive part
• Ordinary ageing: catalysts have a service life and eventually stop converting effectively
• Contamination or overheating from an engine running rich, burning oil or losing coolant, which poisons or melts the substrate

Advisory or failure?

Emission control equipment fitted by the manufacturer that is missing, obviously modified or obviously defective is a major defect. The inspection covers only components that are visible and identifiable, which in practice means the catalytic converter, oxygen sensors and the exhaust gas recirculation valve rather than anything requiring dismantling. There is an age distinction that catches people out: for cars first used on or after 1 September 2002 the tester must reject missing or obviously modified original equipment outright, while earlier cars are only rejected on this where a full catalyst test applies. Separately, an induction or exhaust leak that could affect the emissions readings is itself a major defect. In practice the two failures arrive together — a car without its catalyst will almost always exceed its carbon monoxide and hydrocarbon limits and fail the gas test as well, and if the exhaust is holed badly enough the readings are unreliable and must be retaken once it is repaired.

Can you fix it yourself?

Garage job

Fitting a catalyst is exhaust work under a car, and the specific part matters more than the labour: a universal cat that is not the correct type-approved specification for the vehicle will not bring the readings within limits and will fail again, so buy on the exact engine and emissions standard rather than on price. Most are welded or flanged in, and the fasteners around them are among the most corroded on the car, so plan on cutting something off. Where the old catalyst failed rather than being stolen, replacing it without finding out why is throwing money away — a rich-running engine or an oil leak into the exhaust will kill the new one in the same way. Diagnose the cause first, ideally with gas readings before and after.

What it costs

A type-approved aftermarket catalyst for a mainstream petrol car is £120 to £350 in parts, with one to two hours of labour, putting most jobs at £200 to £500 fitted. Original-equipment parts, close-coupled units bolted to the manifold, and anything on a hybrid or a larger engine take you to the top of the band and above it. Add an oxygen sensor at £40 to £120 if the old one was damaged or is the underlying cause, and expect £50 to £90 of diagnostic time first. On a theft claim, check the insurance position before paying: the excess is sometimes close to the repair cost, and some insurers will fit a marked replacement or a protective cage as part of the settlement.

What happens if you ignore it

The car will not pass, and there is nothing to wait for. Beyond the test, a missing catalyst puts a great deal more carbon monoxide and unburnt hydrocarbon into the air at street level, and it is an offence to use a vehicle on the road with its emission control equipment removed. A car that lost the part to thieves usually needs the connecting pipework and often an oxygen sensor as well, and driving on the cut pipe risks fumes entering the cabin. Where the catalyst is breaking up internally rather than missing, debris travels down the system and can block the silencer, adding another repair to the same bill.

Common questions

Is a missing catalytic converter an MOT failure?

Yes. Emission control equipment fitted by the manufacturer that is missing, obviously modified or obviously defective is a major defect. For cars first used on or after 1 September 2002 the tester must reject a missing or obviously modified original part outright. In practice the car fails twice, because it will also exceed its carbon monoxide and hydrocarbon limits on the gas test.

Can I drive without a catalytic converter?

No. It is an offence to use a vehicle on the road with its emission control equipment removed, and the car will not pass its next test. A vehicle that has lost the part to thieves is usually left with a cut pipe, which risks fumes reaching the cabin, so it wants recovering rather than driving.

Will a universal catalytic converter pass an MOT?

Only if it is the correct type-approved specification for your engine and emissions standard. A cheap universal unit that is not will fail to bring the gas readings inside the limits and the car will fail again, so buy on the exact engine variant rather than on price.

Why did my catalytic converter fail, and will a new one last?

Catalysts have a service life, but early failure usually has a cause: an engine running rich, burning oil or losing coolant will poison or melt the substrate. A rotten-egg smell or a rattle from the casing are the classic signs. Fit a replacement without finding the cause and it will die the same way, so have gas readings taken first.

How much does a catalytic converter cost to replace?

A type-approved aftermarket unit for a mainstream petrol car is £120 to £350 in parts with one to two hours of labour, so most jobs come in at £200 to £500 fitted. Original parts, close-coupled units and anything on a hybrid reach £900 and beyond. Add £40 to £120 for an oxygen sensor and £50 to £90 of diagnostic time. On a theft, check the excess first.

Cars this happens to most

Of every model with at least 2,000 tests in the DVSA data, these record catalytic converter missing or defective on the largest share of their MOTs. Across all of them the typical rate is 0.1% of tests.

1. Subaru Impreza0.6%

   98 times in 16,713 tests

2. Nissan Gt-R0.5%

   11 times in 2,371 tests

3. Toyota Supra0.3%

   6 times in 2,264 tests

4. BMW M50.2%

   6 times in 3,789 tests

5. Peugeot Elddis0.1%

   3 times in 3,312 tests

6. Skoda Kamiq0.1%

   19 times in 24,639 tests

7. Aston Martin DB110.1%

   2 times in 2,611 tests

8. Skoda Scala0.1%

   6 times in 10,179 tests

A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.

Pass rates for every make and model →

The warning light behind this

This defect is what a tester writes down. What you see from the driver’s seat is a symbol, and it is worth checking which one, because manufacturers reuse the same shape for more than one system.

• Engine management lightWhat it means and whether you can keep driving

Fault codes behind this failure

A failure certificate records that the lamp was lit, not what lit it. If this is what your certificate says, one of these codes is usually stored behind it, and reading it is the difference between a diagnosis and a guess.

• P0011 Camshaft timing stuck too far advanced, bank 1
• P0012 Camshaft timing stuck too far retarded, bank 1
• P0014 Exhaust camshaft timing too far advanced, bank 1
• P0021 Camshaft timing stuck too far advanced, bank 2
• P0103 Airflow meter reading too high
• P0108 Manifold pressure sensor reading too high
• P0132 Front oxygen sensor stuck reporting a rich exhaust
• P0136 Rear oxygen sensor after the catalytic converter reading incorrectly
• P0172 Engine running rich — too much fuel for the air going in
• P0201 Injector circuit fault on cylinder 1
• P0300 Engine misfiring, no single cylinder identified
• P0301 Cylinder 1 is misfiring

Look up any of the other 13, or search the full index by code or symptom.

Other common defects

• Direction indicator not working
• Stop lamp not working
• Lamp of the wrong colour, position or intensity
• Brake pipe corroded
• See all defects →