Catalytic Converter

We replace failed and stolen catalytic converters from £300, and we confirm the fault before any part is ordered. A converter reported as failed is often a symptom of a lambda sensor, an exhaust leak or a fuelling fault further up the engine.

The catalytic converter sits in the exhaust between the engine and the silencers. Its ceramic core is coated with platinum, palladium and rhodium, which convert carbon monoxide, unburnt hydrocarbons and nitrogen oxides into less harmful gases. On many modern petrol engines the converter is close coupled to the exhaust manifold, and on some it is built into the manifold itself. Diesel engines use a diesel oxidation catalyst ahead of the particulate filter.

The most common sign is the engine management light with a catalyst efficiency code, usually P0420 or P0430. A converter that has broken up internally can rattle from underneath the car, particularly at idle or when the engine is started. A blocked converter restricts the exhaust, which causes a loss of power under load, poor acceleration and in some cases limp mode. A strong smell of sulphur from the exhaust and a high emissions reading at test are also typical.

A catalyst efficiency code means the engine control unit has compared the upstream and downstream lambda sensor signals and found too little difference between them. That result can come from a converter that has lost its oxygen storage capacity. It can also come from a slow or contaminated downstream sensor, an exhaust leak ahead of the sensor drawing in air, or a misfire sending unburnt fuel into the converter. A misfire left unrepaired will overheat and destroy a new converter within a short period, so the cause has to be corrected first.

We read the codes from every control module and review the misfire counters and fuel trims. We then record both lambda sensor signals on a Picoscope oscilloscope with the engine at operating temperature. A working converter produces a steady downstream signal while the upstream signal switches. We check the exhaust for leaks ahead of the sensors with a smoke leak tester, and where a blockage is suspected we measure exhaust back pressure. You receive a written explanation of the result and a quote, and no work is carried out without your approval.

Converters are stolen for the precious metals they contain, and theft is usually obvious from a very loud exhaust note the moment the engine starts. The vehicle should not be driven far in that condition, because exhaust fumes can enter the cabin. We replace the converter and repair any damaged pipework, oxygen sensors and wiring cut during the theft. If you are claiming on your insurance, you will need a crime reference number from the police.

We can fit a catalytic converter protection device to reduce the risk of theft on the vehicles most often targeted, including the Toyota Prius and Honda Jazz. The device is fitted underneath the vehicle to make the converter significantly harder and slower to remove. Contact us for a price for your vehicle.

Catalytic converter replacement starts from £300. The final price depends on the vehicle, the price of the approved converter for that engine and whether the converter is a separate section or part of the exhaust manifold. Where the fault has not yet been confirmed, an Advanced Diagnostic Assessment costs from £145. Every replacement is covered by our warranty of at least 12 months or 12,000 miles on our work.

We do not replace a catalytic converter with a straight pipe. A vehicle without a working converter is not legal for road use.