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P0420: catalyst system efficiency below threshold

Updated 23 October 2026 · about 8 minutes to read

P0420 is probably the most looked-up fault code there is, and it is also the one most likely to cost you money you did not need to spend. Search it and the first thing you will be told is that your catalytic converter has failed — which is sometimes true, and often not.

The code's official wording is "catalyst system efficiency below threshold, bank 1". Read it carefully: it describes a measurement, not a part. Several very different faults produce that measurement, and only one of them is a dead converter.

What the car is actually measuring

There are two oxygen sensors in the relevant part of your exhaust. The upstream sensor sits before the catalytic converter and feeds the engine computer the information it uses to trim the fuel mixture. The downstream sensor sits after the converter and exists almost entirely to grade the converter's work.

A working converter holds a reserve of oxygen and releases it as needed. The upstream signal swings rapidly up and down as the computer constantly corrects the mixture; the converter smooths all that out, so the downstream signal should be comparatively flat and lazy.

Periodically, the engine computer runs its catalyst monitor and compares the two. If the downstream signal has started to copy the upstream one — swinging in step with it — the converter is no longer storing oxygen the way it should, and the computer stores P0420.

The five causes, cheapest first

Work down this list in order. Each one can produce a genuine P0420 while the converter itself is perfectly healthy, and each is cheaper to check than a converter is to replace.

1. An exhaust leak ahead of the converter

This is the one people skip, and it is the cheapest fix on the list. A cracked manifold, a leaking flange gasket, or a split in the pipe before the downstream sensor lets fresh air into the exhaust stream. The sensor reads that extra oxygen and reports a mixture that never looks right, regardless of what the converter is doing.

How to check: listen with the engine cold and idling — a leak before the converter usually ticks or puffs rhythmically. Look for black sooty staining around joints and welds, which marks where gas has been escaping.

2. A tired or contaminated downstream oxygen sensor

The sensor grading the converter can itself be the problem. Oxygen sensors slow down with age, and they are poisoned by silicone from the wrong sealant, by coolant from a leaking head gasket, and by oil. A sensor that responds sluggishly produces a signal that looks exactly like a failing converter.

Clue: if you also have P0133, P0136, P0139 or a similar O2-sensor code stored alongside P0420, deal with the sensor first and re-test.

3. An untreated misfire or a rich mixture

This is the cause that turns a cheap problem into an expensive one. Unburnt fuel reaching the converter burns inside it, and the heat destroys the ceramic substrate. A misfire you ignored for a few months is one of the most common ways a perfectly good converter dies.

Clue: P0420 alongside P0300, P0301 or any other misfire code means you fix the misfire first. Replacing the converter while the misfire continues will simply cook the new one too.

4. The engine is burning oil or coolant

Worn valve stem seals, tired piston rings or a leaking head gasket send oil or coolant into the exhaust, where it coats the converter's internal surfaces and stops them reacting. The converter is then genuinely failing — but replacing it without fixing the engine just repeats the process.

Clue: blue smoke on start-up or under load, sweet-smelling white smoke, or an oil or coolant level that keeps dropping with no puddle underneath.

5. The catalytic converter really has failed

Only when the four above are ruled out. Converters do wear out — typically well past 100,000 miles — and an aftermarket one fitted earlier in the car's life may simply not have the capacity the original had. Some cars will set P0420 with a cheap replacement converter that is technically working but not to the standard the factory calibration expects.

Check your own O2 sensors and fuel trims CodeGuru shows live sensor values and freeze frame data on any ELM327 adapter.

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How to narrow it down with live data

This is where a scanner that shows live values earns its keep. A code reader that only prints "P0420" leaves you guessing; watching the sensors tells you which of the five causes you are looking at.

Compare the two oxygen sensors

Warm the engine to full operating temperature, hold a steady 2,000–2,500 RPM, and watch both sensors at once:

• Upstream swinging, downstream fairly flat — the converter is doing its job. Look harder at the sensor itself, or at an exhaust leak.
• Both swinging together, in step — the converter is not storing oxygen. Causes 3, 4 or 5.
• Downstream barely moving at all, ever — suspect the sensor is dead or contaminated rather than the converter.

Read the fuel trims

Short-term and long-term fuel trim tell you whether the engine is being fed correctly in the first place. Trims that sit heavily positive point to a lean condition and often an air leak; heavily negative points to a rich condition, which is exactly what cooks converters. Either way, a P0420 with badly skewed trims is not a converter problem yet — it is a mixture problem that will become one.

Read the freeze frame

When the code was stored, the computer saved the conditions at that instant: RPM, engine load, coolant temperature, vehicle speed, fuel trims. That snapshot tells you whether the monitor failed on a cold start, on the motorway, or under hard acceleration — and that narrows the search considerably.

Check for other codes first

P0420 rarely arrives alone when something else is wrong. Scan for every stored and pending code before you decide anything, and if there is a misfire, a lean or rich code, or an O2 sensor code in the list, that is the one to chase. A full system scan is worth running too, since a related fault can sit in a module a basic engine read never reaches.

Can you keep driving?

In the short term, usually yes. P0420 on its own does not normally change how the car drives, and it is not a safety fault. Three caveats:

• It will fail an emissions test. There is no way around that other than fixing the cause.
• If there is also a misfire — especially if the light is flashing — continuing to drive is actively expensive, because that is what kills converters.
• A blocked converter is a different matter. If the car has also lost power, struggles uphill or feels like it is being held back, the converter may be restricting flow, and that needs looking at promptly.

And do not clear the code to pass a test. Clearing resets the emission readiness monitors, and a test centre will reject the car for not being ready — a result that looks worse than the original failure.

Questions people ask

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