A catalytic converter rarely fails on its own. In most cases something else on the vehicle killed it — and if that something isn’t found and fixed before the new cat goes on, the new cat will die the same death. Ten minutes of diagnosis here can save you the cost of a second replacement.
The usual suspects
Unburned fuel — the number one killer
A misfiring engine sends raw fuel down the exhaust. When it reaches the hot catalyst it ignites, and the resulting temperatures melt or crumble the monolith. Worn spark plugs, failing HT leads or ignition coils, leaking or dribbling fuel injectors and faulty MAP sensors are the classic causes. A tell-tale sign on the old unit is a blue or purple tint to the steel shell, or visible melting of the honeycomb — if you see either, the cat overheated, and the cause is upstream of it.
Faulty lambda (oxygen) sensor
The lambda sensor tells the ECU how to trim the fuel mixture. When it fails or reads falsely, the engine can run rich for months, steadily poisoning and overheating the cat. Bear in mind the reverse, too: a mixture fault code doesn’t always mean the sensor itself is dead — it may be reporting a real problem, such as an air leak upstream.
Oil or coolant contamination
A worn engine, failing turbocharger or leaking valve-stem seals let oil into the exhaust, where it coats the catalyst’s precious-metal surface and stops it working. Head-gasket failures do the same with coolant. A cat fouled this way under-performs without any visible damage.
Physical damage
The monolith is ceramic and brittle. Speed bumps, kerbs and debris strikes can crack it — a cracked monolith often announces itself as a rattle. (Check it really is the cat rattling, though: loose heat shields are a common false alarm.) Broken flanges, snapped pipework and blowing flexes usually trace back to worn engine mountings or corroded exhaust brackets putting strain on the system’s weakest point.
Exhaust paste
Exhaust assembly paste used upstream of a cat hardens, breaks up, and shot-blasts the monolith from the inside. It’s also an automatic warranty exclusion on a replacement unit — never use it in front of a cat or DPF. Silicone-based sealant is the safe alternative where a sealant is genuinely needed.
Reading the fault code: P0420
The most common cat-related fault code is P0420 — “catalyst system efficiency below threshold”. It’s also one of the most misdiagnosed. It can mean a genuinely worn-out cat, but it can equally be triggered by a lambda-sensor fault, an exhaust air leak causing false readings, or ignition timing problems. Investigate all of those before condemning the cat — a new converter won’t clear a P0420 whose real cause is elsewhere.
Failed the MOT on emissions?
The emissions printout tells you a lot about whether the cat is really to blame — a high CO reading points at the cat, but high hydrocarbons point at the engine. We’ve covered how to read it in MOT emissions failures explained. Keep the failure report either way: it’s needed for any warranty claim on a replacement.
Fixed the cause? Fit the cure
Once the underlying fault is sorted, find the right replacement with our part finder, then follow our step-by-step fitting guide.