Catalytic converter

What the catalytic converter is

Every petrol, diesel and hybrid car sold in most markets since the 1990s has at least one catalytic converter, often called the catalyst or simply the cat. It looks like a wide section of the exhaust pipe, usually with a heat shield around it, and it is fitted either right after the exhaust manifold, further back under the floor, or in both places.

Inside is a ceramic or metal honeycomb with thousands of thin channels. The channels are coated with small amounts of precious metals, typically platinum, palladium and rhodium. These metals are the catalyst: they make chemical reactions happen at exhaust temperatures without being used up themselves. The honeycomb gives the gases a very large surface to touch on their way through.

The converter only works properly once it is hot, which is why modern engines run a slightly different mixture right after a cold start to heat it up quickly, and why many cars place the first converter as close to the engine as possible.

How it cleans the exhaust

A petrol engine's exhaust contains three main pollutants: carbon monoxide, unburnt hydrocarbons (fuel that did not burn) and nitrogen oxides. A three-way converter, the type used on petrol cars, deals with all three at once. It oxidises carbon monoxide and hydrocarbons into carbon dioxide and water, and reduces nitrogen oxides back to nitrogen.

For this to work, the mixture of air and fuel has to stay very close to the ideal ratio. That is the job of the oxygen sensors. The sensor before the converter tells the engine computer whether the mixture is rich or lean, so it can adjust the fuel constantly. The sensor after the converter watches what comes out. When the converter is healthy, the second sensor's signal is steady, because the converter stores and releases oxygen and smooths out the swings.

Diesel cars use a different set of parts: an oxidation catalyst, usually a diesel particulate filter, and on many newer cars a system that injects a urea solution to break down nitrogen oxides. The principle is the same, but the parts and their faults are different from a petrol car's.

Why catalytic converters fail

A converter has no moving parts and, on a well-kept engine, it can last a very long time. When one fails early, the cause is almost always upstream, in the engine. Replacing the converter without fixing that cause usually means the new one fails as well.

The most common cause is unburnt fuel reaching the converter. When a cylinder misfires, the fuel that should have burnt in the engine burns inside the converter instead, and the temperature can rise high enough to melt the honeycomb. That is why a flashing check engine light, which signals a severe misfire, needs attention straight away.

Other causes include an engine that burns oil or leaks coolant into the cylinders, which coats the channels with deposits, a mixture that stays too rich because of a faulty sensor or injector, and physical damage from hitting a kerb or a deep pothole, which can crack the ceramic inside.

• Misfires that send unburnt fuel into the exhaust.
• Oil burning or coolant leaking into the cylinders.
• A mixture that stays too rich for a long time.
• Impact damage that cracks the honeycomb.

Signs the converter needs attention

The most common sign is the check engine light with code P0420 (bank 1) or P0430 (bank 2), which means the engine computer has measured that the converter's efficiency is below its threshold. These codes do not always mean the converter itself is bad: a faulty rear oxygen sensor, an exhaust leak near a sensor or an engine problem can produce them too, so they need diagnosis rather than an immediate replacement.

A converter that has broken up inside can rattle, especially at idle or when you start the engine. One that is blocked restricts the exhaust, so the car feels weak, struggles on hills and may overheat the exhaust. A strong smell of rotten eggs from the exhaust often points to a rich mixture that the converter cannot handle.

If the car will be presented for a periodic technical inspection, a converter that no longer works properly can also make the car fail the emissions part of the test.

Theft, aftermarket parts and buying a used car

Because of the precious metals inside, original converters are valuable, and in many countries they are a target for thieves. If a car suddenly sounds far louder than usual on start-up, a missing converter is one of the things to check.

When a converter needs replacing, a shop may offer an original part or an aftermarket one. Ask what standard the replacement is made to and whether it is meant for your exact engine, because the engine computer monitors the converter's efficiency and a part that does not meet it will bring the warning light back. Removing the converter altogether is not a repair: the light stays on, emissions rise and the car may not pass inspection.

When you buy a used car, especially an imported one, check whether the converter is still in place and whether any emissions codes are stored. A VIN check on Junto shows the car's recorded history, and a pre-purchase inspection at a shop can confirm the exhaust is intact.

What to ask the shop

If a shop recommends a new catalytic converter, ask what tests confirmed it: the oxygen sensor readings before and after the converter, a check for exhaust leaks, and whether any misfire, oil burning or rich-mixture fault has been ruled out or fixed first. A good shop will explain why the old converter failed, not only that it did.

Ask what the replacement is, how the shop will confirm the repair (usually a test drive followed by a scan once the computer has finished its own converter test), and whether any warranty depends on fixing the underlying engine fault. The interval and the exact part for your car are set by the car maker, so check the owner's manual or ask the shop for the specification. On Junto, the shop can send each suggested line with its price for your approval by SMS before the work starts.

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