Catalytic Converter Faults: Causes, Symptoms and MOT Impact

Symptoms of a failing catalytic converter

A converter that is partly blocked restricts the flow of exhaust gas. The engine then struggles to breathe, so the car feels flat, slow to accelerate, or hesitates on hills. In more serious cases it can overheat or stall.

Inside, the honeycomb can break up and rattle, especially at idle or when you tap the casing. Other signs include a smell like rotten eggs, a warning light, higher fuel use and, for diesels and petrols alike, a failed emissions reading at the MOT.

These symptoms overlap with clogged fuel filters, failing ignition parts and other exhaust faults, so do not assume the converter is to blame. A garage can check the back pressure in the exhaust and compare readings before and after the converter, which separates a restriction from some other cause.

• Loss of power and sluggish acceleration
• Rattling from underneath, particularly at low revs
• Engine management or emissions warning light
• Strong sulphur smell from the exhaust
• Excess heat under the car after a run

What damages a catalytic converter

Converters rarely wear out on their own. They are damaged by something else going wrong. Oil burning in the engine, from worn piston rings or valve seals, coats the honeycomb. A coolant leak, often from a failing head gasket, can contaminate and clog it. Misfires send unburnt fuel into the exhaust where it overheats and melts the ceramic.

A faulty oxygen sensor or fuelling fault can also overfuel the engine and wreck the converter over time. That is why diagnosis matters: fitting a new converter to an engine that is still burning oil or misfiring is likely to end the same way.

Theft of catalytic converters

Because of the metals inside, catalytic converters are targeted by thieves, who cut them out from underneath the car in a matter of minutes. The first sign is often a sudden, loud roar when you start the engine, plus a warning light.

If this happens, report it to the police and your insurer, and do not drive the car if the exhaust is loose or damaged. Replacement is possible once the car has been inspected, but we do not give insurance advice, so talk to your insurer about the claim.

Fitting a protective cage or marking the part is sometimes discussed, but whether anything suits your vehicle depends on the car, so ask when it is in for inspection. Parking in a well-lit, busy spot is a common-sense measure that costs nothing.

How emissions diagnostics find the cause

Emissions diagnostics starts with reading the fault codes from the engine's computer, checking live data from the oxygen sensors, and looking at fuel trims. An exhaust gas analyser measures what is coming out of the tailpipe. The results show whether the converter is failing or something upstream is making it fail.

At Banks of the Tyne in Hebburn we carry out emissions diagnostics and can replace catalytic converters when needed. We would rather identify why the part failed than swap it and hope.

Warning lights are worth reading rather than clearing. Switching a light off with a code reader does not fix anything, and the fault will return, so the stored codes are valuable evidence.

MOT impact and your options

The MOT includes an emissions test, and a failed or missing catalytic converter on a car that was built with one can lead to a fail. The tester also checks for exhaust leaks and visible damage. Check the current MOT rules for the details that apply to your vehicle.

Depending on the diagnosis, the answer may be a repair to the engine or sensor, a replacement converter, or a repair to the exhaust. Cost varies with the make, model and which type of part is required, so ask for an estimate after inspection.

Frequently asked questions