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Catalytic converters
The catalytic converter cleans the exhaust gases, turning most of the harmful content into carbon dioxide, nitrogen and water.
What catalytic converters means
It contains precious metals, which is why they are sometimes stolen from under parked cars. A failed one shows up either as an emissions failure or as a stored fault from the sensors either side of it.
What the job covers
- Catalytic converter replacement to type approval
- Emissions testing before and after
- Cat theft repair and shielding
- Lambda sensor and fuelling faults
- Downstream sensor diagnosis
When you might need it
- An emissions failure at the MOT.
- A rattle from underneath, which can be the internal honeycomb breaking up.
- A sudden loud exhaust noise after the car has been parked, which can mean the converter has been cut out and stolen.
What it costs, and what changes the price
Typically £300 to £1,200.
- Type approval matters. A converter must meet the standard for your car, and a cheap unapproved one may fail the test.
- The sensors either side are sometimes replaced at the same time.
- A protective cage or marking, fitted after a theft, is priced separately.
How long it takes
Usually two to three hours, longer if bolts have seized.
What the MOT data shows
Out of 42,728,066 MOT tests in 2025, these are the parts covered by this job that testers recorded as faults.
| Part | Recorded in 2025 |
|---|---|
| Catalyst emissions | 436,271 |
Common questions
Why do people steal them?
For the precious metals inside. Higher hybrids and some older models are targeted more often because their converters contain more.
Will a cheap replacement pass the MOT?
Only if it is type approved for your vehicle. It is worth confirming that before fitting rather than after failing.
Can a blocked converter be cleaned?
Sometimes a chemical clean helps a partly blocked one. A broken up internal structure needs a new unit.