Will P0420 fail an MOT?

Yes, on two counts. The engine management lamp it switches on is a major defect for petrol cars first used on or after 1 July 2003 under section 8.2.1.2 of the MOT inspection manual, and the car fails on the lamp whatever else it does. A worn catalytic converter can also fail the emissions test itself, because carbon monoxide and hydrocarbons rise and the lambda reading at fast idle may fall outside the required 0.97 to 1.03 band. Cars first used before July 2003 are not failed on the lamp but can still fail on the gas readings.

Can I drive with a P0420 code?

Generally yes. The fault does not make the car unsafe and it usually drives normally. Two things should change your mind: a rattling noise from under the car, which means the ceramic substrate is breaking up and can restrict the exhaust, and a flashing engine light or rough running, which means a misfire is dumping raw fuel into an already struggling converter. The car cannot pass an MOT until it is fixed.

Does P0420 always mean I need a new catalytic converter?

No, and assuming it does is an expensive mistake. The code describes what the two oxygen sensors report about the converter, not the converter itself. A worn rear oxygen sensor, an exhaust leak ahead of that sensor, or an engine running rich will all produce the same reading from a perfectly serviceable converter. A garage can distinguish them in minutes by watching the sensors live, which is worth doing before authorising one of the dearer jobs on an ordinary car.

Will replacing the oxygen sensor fix P0420?

Sometimes, and it is worth investigating first because it is a far smaller job. If the rear sensor has become slow or contaminated it will misreport a healthy converter and a new sensor clears the code for good. If the converter really has lost its oxygen storage, a new sensor will simply report that fact more accurately and the code returns after the next monitored drive. Live data from both sensors tells you which situation you are in before any parts are bought.

Powertrain code · standard

P0420Catalytic converter not working efficiently enough

Standard definition: Catalyst System Efficiency Below Threshold (Bank 1)

Last updated 27 July 2026

Quick answer

The engine control unit has compared the oxygen sensors either side of the catalytic converter and concluded the converter is no longer cleaning the exhaust well enough. It is often the converter itself, but a tired sensor or an exhaust leak produces the same code, and replacing the converter without checking those is how people spend a great deal of money twice.

How the car works this out

A healthy catalytic converter stores oxygen and releases it as the mixture swings, which smooths the exhaust chemistry passing through it. The sensor in front of the converter therefore switches rapidly between rich and lean several times a second, while the sensor behind it should sit fairly flat and lazy. The engine control unit counts how much the rear sensor mimics the front one over a monitored drive. When the rear signal starts copying the front, the converter has stopped storing oxygen, and once that ratio crosses the threshold the code is set and the lamp comes on.

What you might notice

Usually nothing at all — most people find out from the lamp or a code read
A sulphurous, rotten-egg smell from the exhaust, especially under load
A rattle from under the car at idle or over bumps, if the internal substrate has broken up
Slightly worse fuel consumption and a little less urgency at higher revs
On some cars a second lamp or a mild loss of power if the converter is physically blocking

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The catalytic converter itself has degraded, which happens with age, heat cycles and mileageCommon
How to check: With the engine warm and the codes read live, watch the rear sensor. If it swings up and down roughly in step with the front sensor rather than holding a steady voltage, the converter is no longer storing oxygen.
2The rear oxygen sensor is aged or contaminated and reporting the converter unfairlyCommon
How to check: Compare the rear sensor's behaviour when the mixture is forced rich by a snap of the throttle. A sensor that is slow to respond, or that never leaves a narrow band, is suspect in its own right.
3An exhaust leak between the manifold and the rear sensor, drawing air in and making the exhaust look cleaner or leaner than it isPossible
How to check: Listen with the engine cold for a ticking or puffing that quietens as the joints expand and seal. Check the manifold gasket, the flexi joint and the downpipe clamps.
4The engine has been running rich or misfiring and has cooked the converterPossible
How to check: Look for stored misfire or rich-running codes alongside it. A converter that failed because of another fault will fail again if only the converter is replaced.
5An aftermarket or sports catalytic converter with less substrate than the originalPossible
6Oil or coolant burning through the engine and poisoning the substrateLess common

What to do about it

1Do not buy a catalytic converter on the strength of this code alone. It is the single most commonly over-replaced part in the fault code world, and the diagnosis is a live data check that takes a competent garage minutes.
2Rule out exhaust leaks first, because they are cheap by comparison and they produce this code on a perfectly good converter.
3Check whether any other codes are stored. A misfire or a rich-running fault is both the likely reason the converter died and the reason a new one will not last.
4Get the rear oxygen sensor assessed before condemning the converter. A lazy sensor is a fraction of the job.
5If the converter genuinely is finished, use a proper type-approved replacement. A cheap universal unit will often set the same code again within months, and the fitting record matters if the car is ever inspected.

Can you keep driving?

Yes

A P0420 on its own does not make the car unsafe and most people drive on one for months without noticing anything. The two things that change that are a rattle, which means the substrate is breaking up and pieces can restrict the exhaust, and any accompanying misfire, which will destroy what is left of the converter quickly. Neither changes the fact that the car cannot pass an MOT with the lamp lit.

Will it fail an MOT?

This code fails cars two separate ways. The lamp alone is a major defect on petrol cars first used on or after 1 July 2003, so the test ends in a failure certificate however well the car drives — that single defect accounts for 371,859 failures in the DVSA data this site holds. Separately, the converter is the part that brings the tailpipe gases inside the limits, so a genuinely spent one pushes carbon monoxide and hydrocarbons up at fast idle and can drag the lambda reading outside the 0.97 to 1.03 window the tester needs to see. And if a previous owner solved the problem by cutting the converter out, that is its own defect: a tester will fail a car whose catalytic converter is missing when one was fitted as original equipment. Clearing the code before the test does not help, because the catalyst monitor then reads as not ready and the emissions check cannot be completed.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Will P0420 fail an MOT?: Yes, on two counts. The engine management lamp it switches on is a major defect for petrol cars first used on or after 1 July 2003 under section 8.2.1.2 of the MOT inspection manual, and the car fails on the lamp whatever else it does. A worn catalytic converter can also fail the emissions test itself, because carbon monoxide and hydrocarbons rise and the lambda reading at fast idle may fall outside the required 0.97 to 1.03 band. Cars first used before July 2003 are not failed on the lamp but can still fail on the gas readings.
Can I drive with a P0420 code?: Generally yes. The fault does not make the car unsafe and it usually drives normally. Two things should change your mind: a rattling noise from under the car, which means the ceramic substrate is breaking up and can restrict the exhaust, and a flashing engine light or rough running, which means a misfire is dumping raw fuel into an already struggling converter. The car cannot pass an MOT until it is fixed.
Does P0420 always mean I need a new catalytic converter?: No, and assuming it does is an expensive mistake. The code describes what the two oxygen sensors report about the converter, not the converter itself. A worn rear oxygen sensor, an exhaust leak ahead of that sensor, or an engine running rich will all produce the same reading from a perfectly serviceable converter. A garage can distinguish them in minutes by watching the sensors live, which is worth doing before authorising one of the dearer jobs on an ordinary car.
Will replacing the oxygen sensor fix P0420?: Sometimes, and it is worth investigating first because it is a far smaller job. If the rear sensor has become slow or contaminated it will misreport a healthy converter and a new sensor clears the code for good. If the converter really has lost its oxygen storage, a new sensor will simply report that fact more accurately and the code returns after the next monitored drive. Live data from both sensors tells you which situation you are in before any parts are bought.

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