Catalyst: how long it lasts and how to tell it is dead

P0420 is not a death sentence. What kills a catalyst, how to tell a dead one from a wrongly blamed one, and what actually follows a «delete».

P0420 — «catalyst efficiency below threshold» — is one of the commonest codes and one of the most expensively misread. Half the cars carrying it have a perfectly good catalyst.

How long it lasts

A factory catalyst is designed for roughly 150,000–200,000 km and, on a healthy engine, often lasts longer. It barely wears out by itself: what kills it is what reaches it.

What shortens its life:

• Misfires. Unburnt fuel finishes burning inside the catalyst and melts the matrix. A flashing light means exactly that, which is why you must not drive with it.
• A rich mixture — P0172, P0175. The same mechanism, only slower.
• Oil in the combustion chamber. Worn valve seals and rings, and the catalyst clogs with ash.
• Knocks from below. Ceramic matrices crack on speed bumps as readily as they age with mileage.

Hence the rule: a catalyst dies of somebody else's disease. Replacing it without curing the cause means buying a second one for the same lifespan.

Telling a dead one from a wrongly blamed one

The unit sets P0420 by comparing two oxygen sensors, before and after the catalyst. If the rear sensor lies, the conclusion is wrong while the catalyst is fine.

What to check before buying the part:

• The rear oxygen sensor. A stuck signal — P2270, P2271 — is a very common reason for a false accusation. Its signal should be smooth, not a copy of the front one.
• An exhaust leak between catalyst and sensor. Extra air gives the same picture. Found by ear and by soot around the joints.
• Warm-up. The separate code P0421 means «below par specifically during warm-up» and often points at a sensor heater rather than the catalyst.
• The state of the engine. If misfire or mixture codes are present, fix those first and only then see whether P0420 remains.

Signs of a genuinely dead catalyst: a noticeable loss of pull, the engine choking at high revs, a hydrogen sulphide smell, a rattling matrix when the housing is tapped.

Why it is not cut out

The subject of removal comes up almost every time, and the honest answer is: we do not help with it and we do not explain how to fool the rear sensor. The reasons are practical rather than moral.

• The code will come back. The unit goes on comparing the sensors and sets P0420 again. Spacers and remapping are separate money and separate trouble.
• An emissions test cannot be passed. The test sees it, and incomplete monitors give away a fresh clearing.
• The car becomes harder to sell. The buyer arrives with a scanner, and a missing catalyst shows.
• The exhaust becomes more poisonous — not for you, but for whoever is behind you and standing beside you at the lights.

If the catalyst really is dead, there are two sensible paths: replace it, or fit a simpler approved one where that is allowed. Both are a conversation with a garage, not with an app.

In short

• A catalyst dies of somebody else's disease: misfires, rich mixture, oil, knocks.
• P0420 is not a verdict: check the rear oxygen sensor and exhaust leaks first.
• Replacing it without curing the cause buys a second one for the same lifespan.
• We do not help to cut it out or fool the sensor: the code returns, the test fails, and the car is harder to sell.