You turn the key and the car sounds like it has been fitted with a straight pipe. Enormously loud, deep, wrong. Nothing else is obviously damaged. There is no broken glass, the doors were locked, and for a few seconds it is genuinely hard to work out what has happened.
Then you look underneath and there is a gap in the exhaust where a component used to be.
How to be sure that is what it is
The noise is the giveaway, and it is not subtle — it is not the gradually worsening blow of a corroded exhaust, it is instant and severe. Alongside it you will usually get an engine management light, because the sensors either side of the converter are now reporting something impossible.
Underneath, you will see cleanly cut pipe ends rather than a rusted-through joint. Sometimes the sensors have been cut through too and are hanging loose.
That is different from a converter that has failed, which happens gradually, sets a fault code and tends to bring the car into limp mode rather than making it deafening. If the change was instant and the car is loud, it is theft.
Do not drive it, beyond getting it somewhere safe
There are three reasons, and only one of them is legal.
The MOT inspection manual treats manufacturer-fitted emission control equipment that is missing, obviously modified or obviously defective as a major defect. That is a fail, and using the car on the road in that state is not something to do casually for a fortnight while you sort out a claim.
The exhaust is also now leaking gas well ahead of where it is meant to leave the car, and the cut ends can be close to things that do not enjoy heat. And the noise alone will attract exactly the attention you would rather not have.
Where it is worth calling us: if the car needs to get from where it was parked to a garage without being driven. A flatbed avoids the legal problem, the noise and any risk from a cut exhaust, and we cover Sheffield and South Yorkshire day and night — call DK’s Recovery on +44 7867 860723. Our guide to getting a car to a garage when it cannot be driven explains what that involves.
The first hour
Photograph it before anything is touched. Underneath, the cut ends, the location, and the surroundings. Your insurer will want this and so may the police.
Report it to the police and get a crime reference number. Do this even if you do not expect anything to come of it. The reference is what the insurance claim runs on, and the reporting is what makes local patterns visible.
Ring your insurer before ordering parts. Comprehensive policies normally cover theft of parts, and they will want to have a view on where the work is done.
Check for cameras. A neighbour’s doorbell, a shop, a car park. Footage disappears on a rolling delete, so ask early rather than next week.
Why this happens at all
A catalytic converter contains small quantities of platinum, palladium and rhodium. Those metals are worth a great deal by weight, and a converter is a bolted, cuttable component sitting in the open air underneath a parked car.
That is the whole economics of it. It also explains the targeting. Hybrids are hit hard because their converters spend less time at full working temperature and retain more of the metal. Older petrol cars can carry higher loadings than newer ones. Anything with ground clearance is easier to get under, which is why vans and taller vehicles feature heavily and why a low car is a slightly less attractive proposition.
None of that is about the value of the vehicle. A twelve-year-old hybrid can be a better target than a new car parked next to it.
Replacement
Expect the garage to quote for the converter itself plus whatever sensors and pipework were cut, and expect a genuine part and an aftermarket one to be a long way apart on price. Aftermarket converters vary in quality, and a cheap one that will not satisfy the sensors becomes a recurring fault rather than a repair.
Supply can be slow on some models, which is worth knowing before you arrange a hire car for two days.
Making your car a worse choice
You cannot make it impossible. You can make it slower and less rewarding than the car parked further along.
Park to block the underside. Against a wall or a fence, tight to a high kerb, between other vehicles, in a garage where you have one. Overlooked and well lit beats quiet and dark.
Fit a shield or cage. Purpose-made guards exist for most commonly targeted models. They are not impenetrable, but they add time and noise.
Have the converter forensically marked. Marking makes a stolen unit traceable and harder to sell on, which is the point at which the whole business stops working. Some police forces and community schemes run marking events.
Fit a tilt sensitive alarm if the car does not have one. Jacking the car is part of the job.
The buying end
The trade in stolen converters exists because someone buys them. Under the Scrap Metal Dealers Act 2013 a dealer in England and Wales cannot pay cash for scrap and has to record and verify who they are buying from. It is the same set of rules that governs scrapping a car properly, and for the same reason — traceable payment and identified sellers are what make stolen metal difficult to shift.
Frequently asked questions
What does it sound like if my catalytic converter has been stolen?
Extremely loud, and immediately obvious. The engine roars rather than hums, the noise rises sharply with the throttle, and it sounds more like a race car with a hole in the exhaust than a fault developing. There is usually no other damage to see, which is why people look under the car and find a gap where a component used to be.
Is it legal to drive with the catalytic converter missing?
No, not on the road. The MOT inspection manual treats emission control equipment fitted by the manufacturer that is missing, obviously modified or obviously defective as a major defect, which is a fail. The car is also likely to be over the noise limit, and exhaust gas escaping ahead of its normal exit point is a reason not to sit in it with the engine running.
Does insurance cover catalytic converter theft?
Comprehensive policies normally cover theft of parts, and third party fire and theft may too, but the excess can be a meaningful share of the repair on a cheaper car. Report it to the police first and get a crime reference number, because your insurer will ask for it.
Which cars get targeted?
Hybrids and older petrol cars are hit disproportionately, along with anything that sits high enough to get under easily. Vans and taller vehicles are attractive for the same reason. It is driven by the value of the precious metals inside the converter, not by the value of the car, which is why a modest older hybrid can be a bigger target than a new saloon.
How quickly does it happen?
Fast enough that noise and time are not much of a deterrent. It is commonly reported as taking a couple of minutes in a car park in daylight, which is why prevention is about making a specific car a worse choice rather than about anyone getting there in time.
What actually reduces the risk?
Parking so the underside is hard to reach — against a wall, kerbside wheels in tight, in a garage or a lit and overlooked spot. A protective cage or shield fitted to the car. Forensic marking of the converter, so it is traceable and worth less to a buyer. A tilt sensitive alarm. None of it is absolute, and all of it moves you down the list.
