The catalytic converter: what it does, and what kills one

A catalytic converter is a ceramic honeycomb, coated with precious metals, sitting in the exhaust. Gas passes through it and comes out chemically different. There are no moving parts, nothing to service, and nothing to adjust.

Which is why the single most important thing to know about it is this: a converter that has failed was almost always killed by something else. Replace it without finding what killed it and you will replace it again.

What the chemistry actually is

A petrol engine's three-way catalyst does three reactions at once, which is where the name comes from:

• Carbon monoxide is oxidised to carbon dioxide.
• Unburnt hydrocarbons are oxidised to carbon dioxide and water.
• Oxides of nitrogen are reduced back to nitrogen and oxygen.

The third one is the difficult one, and it is the reason the oxygen sensor exists. Oxidation needs spare oxygen; reduction needs the opposite. A three-way catalyst can only do both at once inside a very narrow band of air-fuel ratio around stoichiometric — roughly 14.7:1 for petrol. Outside it, one half of the job stops. That narrow window is what the fuelling system is really chasing, and it is why a converter cannot do its job on an engine that is running rich or lean.

The Persian original states that a diesel catalyst converts carbon monoxide, oxides of nitrogen and hydrocarbons into carbon dioxide, nitrogen and water. That is not right, and the reason it is not right matters. A diesel oxidation catalyst runs in exhaust with a large oxygen surplus, and in that condition NOx cannot be reduced — there is nothing for it to give its oxygen to. A DOC deals with carbon monoxide and hydrocarbons only. Removing NOx from diesel exhaust needs separate equipment: a NOx trap, or selective catalytic reduction injecting urea. That is the entire reason those systems exist and why they cost what they do.

How long one should actually last

The original article gives three different lifespans in the space of one paragraph — 50,000 to 60,000 km, then 80,000 km, then about four years — and all three are far too low.

On a healthy engine a converter should last 160,000 km or more, and very often the life of the car. Original converters are routinely still working at well past that.

This matters because the low figure invites people to replace a working part on age alone, and — worse — to accept a failed converter at 60,000 km as normal wear when it is a symptom. A converter that dies early is reporting a fault upstream of it.

What kills them

Three mechanisms, and they leave different evidence.

Melting and blockage from unburnt fuel. This is the big one. A misfiring cylinder pumps raw fuel into the exhaust, which burns inside the converter instead of in the engine. Temperatures climb far beyond what the ceramic is designed for, and the honeycomb slumps and partially fuses. The converter is now a restriction, and the engine loses power at high load — a car that feels fine at low speed and will not pull up a hill.

So an ignition misfire is not just a rough-running problem. A misfire left unrepaired destroys the converter, which is why a flashing check engine lamp means stop rather than continue — that specific signal means a misfire severe enough to be doing damage right now.

Poisoning. Coolant from a failed head gasket, or oil past worn valve stem seals or rings, coats the catalyst surface and blocks the reaction sites. The coating can be chemical rather than physical, so the converter looks fine and does nothing. Leaded fuel destroys one outright, and silicone from the wrong sealant used on an intake gasket will do it too.

Mechanical damage. The ceramic monolith is brittle. A hard impact — a speed bump taken at the wrong angle, a dropped exhaust — cracks it, and the pieces rattle. A converter that sounds like gravel when you tap it has broken up internally, and the fragments can block the rest of the exhaust.

Recognising a failed one

Blocked is the easiest to identify: progressive power loss that gets worse the harder you ask, poor top-end, sometimes a smell of sulphur and visible heat glow. The engine is being throttled by its own exhaust. A back-pressure gauge in the oxygen sensor port measures it directly; a vacuum gauge showing manifold vacuum falling steadily at a held engine speed says the same thing.

Chemically dead is harder, because the car drives normally. It shows up as P0420 or P0430 — catalyst efficiency below threshold — which the ECU determines by comparing the upstream and downstream oxygen sensors. A working converter buffers oxygen, so the downstream sensor should be comparatively slow and steady while the upstream one swings. When the downstream sensor starts mirroring the upstream one, the converter has stopped storing oxygen.

P0420 is one of the most misdiagnosed codes there is. It does not say the converter has failed; it says the comparison failed. A lazy downstream sensor, an exhaust leak ahead of it drawing in air, or an engine running outside the stoichiometric window will all produce it with a perfectly good converter underneath. Check the fuel trims and the sensor before condemning the part — it is by far the most expensive thing in that circuit.

Removing it

The original article edges toward the topic of taking the converter out. It is worth being direct instead.

It is illegal in most places, including under Iran's own vehicle inspection regime — catalyst function has been part of the technical inspection since 1391, and a car without one does not pass. It also will not fix anything: on a healthy engine a converter costs a barely measurable amount of power, and a converter restrictive enough to be worth removing is one that an engine fault already destroyed. That fault is still there.

And it fails the ECU's own test. Take the converter out and the downstream sensor immediately mirrors the upstream one, which is exactly the P0420 pattern above — so the light comes back on, permanently.

Related codes

P0420 and P0430 are catalyst efficiency below threshold for bank 1 and bank 2 respectively. They are among the most-read pages in this corpus, which is a fair indication of how often the part gets blamed.