Catalytic converter

The converter is part of the exhaust, usually close enough to the engine to heat quickly. Exhaust gases pass through a coated honeycomb inside its metal casing. On a modern petrol car, that coating helps turn carbon monoxide, unburned hydrocarbons and nitrogen oxides into less harmful gases. The oxygen sensors and engine management system help hold the mixture where the converter can do its job. It is an emissions component, rather than a silencer, although a stolen or fractured unit also makes the exhaust loud.

The unit may be a short section under the car, or built into the exhaust manifold in a crowded engine bay. The shape matters to the bill. A bolt-on underbody section can be easier to exchange than a manifold assembly with shields, seized fixings and sensors attached. Ask the garage to identify the part on the vehicle, not just quote a generic converter from a catalogue.

A sudden, very loud exhaust note after parking can mean that the converter or another section of pipe has been cut away. Look from a safe position beside the car for a fresh gap and cut pipe ends; do not crawl underneath a car resting only on a jack. A leak may also smell of exhaust fumes near the floor or enter the cabin. The DVSA treats a major exhaust leak as an MOT defect and treats fumes entering the cabin separately, so this needs prompt attention.

A failing internal honeycomb may rattle, especially when the engine starts or changes speed. If it breaks up and restricts the exhaust, the engine can feel unwilling to rev. A warning light can mean the monitored emissions system is unhappy, but it does not prove the converter itself is exhausted. Bosch's diagnostic guidance on lambda and air mass sensors lists air leaks among the possible causes of a sensor fault and tells technicians to rule out ignition and fuel supply faults. The scan is a starting clue, not a parts order.

Yes, where the car should have the equipment. DVSA manual section 8.2.1.1 tells testers to inspect visible and identifiable emissions components, including catalytic converters and oxygen sensors. A manufacturer's emissions-control component that is missing, obviously modified or obviously defective is a major defect. The manual tells testers to reject vehicles first used on or after 1 September 2002 in that condition. Earlier vehicles are assessed using the manual's emissions-test flowcharts, because not every older vehicle is subject to the full catalyst test.

Section 8.2.1.2 covers the measured exhaust gases and the engine malfunction lamp. A car can fail for emissions over its applicable limit even when the converter casing looks complete. Conversely, the presence of a warning code alone does not tell you which part to buy. The tester records the actual defect, which can be a failed gas reading, a missing emissions component, a leak that could affect emissions or a malfunction indication, depending on what was found.

Begin with the reason the car came in. Tell the garage when the noise or light appeared, whether the car has lost power, and whether anyone has already cleared a fault code. Show the MOT printout if the test failed. The mechanic can then inspect the pipework for leaks or theft damage, read stored faults, check the sensor readings and examine the engine's running. An emissions reading after an obvious leak has been fixed is more useful than one taken through a damaged exhaust.

Ask the repairer to separate the fault diagnosis from the repair authorisation. If the quoted job is a converter, ask which observation rules out a leaking flexi pipe, a sensor, ignition trouble or bad fuelling. A code associated with poor catalyst efficiency describes what the computer inferred from sensor signals; it cannot by itself prove the honeycomb is the cause. If an engine fault is feeding unburned fuel or oil into the exhaust, ask how that fault will be corrected before the new unit goes in.

The fitted range in the site's repair data is £175 to £900 for a mainstream car. It describes observed UK repair prices, not a promise that every vehicle will fit inside it. The same entry gives a published labour allowance of 1.2 hours for a straightforward job. A high quote may be justified by a manifold-integrated unit, awkward access, seized fasteners or an expensive vehicle-specific part. A low quote may cover a simpler underbody section with reusable sensors and sound adjacent pipes.

Parts quality matters because an emissions repair must still pass the applicable MOT test. Ask what converter specification the garage proposes and whether it is correct for the car. The invoice should say what was fitted, not just list a vague exhaust part. If the old unit was stolen, the repair may also need a section of pipe, sensor wiring or heat shielding. That is a different scope from unclamping a failed but complete unit.

A quote can also include diagnosis, exhaust-gas testing and a retest. Those charges are separate decisions. Bromley Trading Standards advises asking for a written quotation where the trader can fix a price, or a written estimate if they cannot, and an itemised invoice showing parts and labour. Set an agreed ceiling for extra work so a corroded joint does not silently turn the job into a larger bill.

A capable home mechanic can replace some bolt-on exhaust sections with the right lifting equipment and the vehicle maker's fitting instructions. The hazards are substantial: the exhaust becomes very hot, the car must be supported securely, sensors and wiring are easily damaged, and corroded fixings can snap. Cutting a stolen-unit repair into place also calls for a sound, gas-tight joint. Working under a jack alone is unsafe, even for a quick look.

DIY makes most sense only when the unit and root cause are clear, access is safe and the correct part has been identified. For a manifold-integrated converter, repeated sensor faults or a stolen unit with damaged wiring, a garage has the tools to measure the problem and provide a documented repair. The MOT tests the finished vehicle, whoever fitted the part.

Read the result before booking parts. GOV.UK says major and dangerous defects fail the test, while minor and advisory entries can remain on a passing record. An emissions failure may name the gas limit, the malfunction lamp, a missing component or a leak. Give the repairer the precise wording and ask for evidence that its proposed work addresses that recorded defect. If a diagnosis contradicts the MOT result, discuss it with the original test centre before repairs alter the evidence.

For a retest, the general GOV.UK rule is a partial retest when the car returns to the same centre within 10 working days after being taken away for repair; the centre may charge a partial fee. If the vehicle stays at that centre for repair and is retested within that period, the partial retest is free. Beyond the window, a full test and full fee apply. Converter repairs do not appear in the special next-working-day free-item list.

If you think the failed result itself is wrong, GOV.UK says to raise it with the test centre before any work begins and appeal to DVSA within 14 working days of the test. Do not repair the disputed item until the appeal process finishes. If you accept the failure, get the car repaired promptly and check that it remains roadworthy before any permitted journey. An existing MOT certificate is not permission to drive a dangerous vehicle.