Catalytic converter fault: is it the converter or the oxygen sensor?

The engine warning light is on and the diagnostic equipment reports a problem with the catalytic converter. Does that mean you need a new one? Not necessarily. Below you can read how the catalytic converter and the oxygen sensors work together, which other causes give the same fault message and how we find out what is really wrong.

By the team at Autobedrijf Willemsbrug, RDW-approved APK (MOT) test station since 1979 · Updated 6 November 2026

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The short answer

A fault message about the catalytic converter does not automatically mean the converter is broken. A leak in the exhaust, an oxygen sensor or an engine fault can also be the cause. According to the ANWB (the Dutch motoring organisation), the engine warning light also comes on when one of the oxygen sensors is not working properly. That is why we first read the fault codes and take measurements before we replace a catalytic converter or an oxygen sensor.

Exhaust repairs at Autobedrijf Willemsbrug: we first check whether a repair is enough. We weld, replace a single part or make a section to measure, and we replace catalytic converters and oxygen sensors after a diagnosis. We do not clean, replace or remove diesel particulate filters. We tell you the price in advance based on your registration number. Call +31 10 479 00 00. You bring the car and we call you when it is ready. A loan bike is free while one is available. Bringing the car on a Saturday is possible by arrangement. Our team speaks English. More about exhaust repairs

How do the catalytic converter and the oxygen sensors work together?

The catalytic converter makes the exhaust gases less harmful, and the oxygen sensors keep an eye on how well it does that. An oxygen sensor, also called a lambda sensor, is a sensor in the exhaust that measures how much oxygen there is in the exhaust gases. The engine computer uses that measurement to adjust the mix of air and fuel.

Inside the catalytic converter, according to the ANWB, the exhaust gases come into contact with precious metals such as platinum, palladium and rhodium. This turns harmful substances into less harmful ones. A catalytic converter works best at operating temperature. The closer it sits to the engine, the faster it warms up.

A petrol car usually has two oxygen sensors:

  • The sensor before the catalytic converter measures the mixture coming out of the engine. The engine computer uses this signal to control the fuel.
  • The sensor after the catalytic converter (also called the diagnostic sensor) checks whether the converter is still working well. According to Bosch, the smaller the voltage swing of this sensor, the better the condition of the catalytic converter.

If the catalytic converter stops working well, the signal from the rear sensor looks more and more like the signal from the front one. The engine computer notices this, stores a fault code and switches on the engine warning light. What the different warning lights mean is explained in engine warning light on: meaning and diagnosis.

Why can the same fault message have two causes?

The engine computer judges the catalytic converter through the oxygen sensors. If the signal from a sensor is wrong, or if air gets into the exhaust, the computer may think the converter is working poorly when it is not. The reverse also happens: a broken catalytic converter can give a fault code for a sensor.

  • An oxygen sensor that is not working properly: according to the ANWB, the engine warning light also comes on when one of the oxygen sensors is not working properly. Bosch calls the oxygen sensor a wear part and advises having it checked every 30,000 km.
  • A leak in the exhaust: Bosch names an air leak in the intake or the exhaust, before the sensor, as a possible cause of an oxygen sensor fault code. Walker (Tenneco) writes that an exhaust leak can affect both the catalytic converter and the sensors.
  • An engine that does not burn its fuel properly: misfires or clogged injectors change the make-up of the exhaust gases. Bosch names blocked injectors as a possible cause of an oxygen sensor fault code.
  • A catalytic converter that really is worn or damaged: Bosch also lists a defective catalytic converter itself as a cause of an abnormal oxygen sensor signal.

Bosch puts it like this: fault codes for the oxygen sensor or the mixture control do not in every case mean that the oxygen sensors are faulty. The same applies to the catalytic converter. A fault code points to a system, not to a part.

What does fault code P0420 mean?

Code P0420 means that the engine computer considers the efficiency of the catalytic converter too low. It is a message about how the converter performs, not proof that it is broken. Walker, a manufacturer of exhausts and catalytic converters, writes that a P0420 code does not in every case mean the converter has to be replaced.

With code P0420, we first look at leaks, the ignition and the sensors. A leak before the rear sensor, a spark plug that does not spark properly or a sensor that reacts slowly can give the same code. If the catalytic converter is replaced without fixing that cause, the message can come back.

First the cause, then the part. Walker writes that catalytic converters are designed to last as long as the car. If one fails anyway, there is usually a reason. If that reason is not found, a new catalytic converter can soon fail as well.

Which signs point to a faulty catalytic converter or oxygen sensor?

The signs are similar. You cannot tell from the symptoms alone which part is at fault. This overview does help you judge what may be going on.

Sign Catalytic converter Oxygen sensor
Engine warning light on Catalytic converterYes, according to the ANWB, when the converter no longer works Oxygen sensorYes, according to the ANWB and Bosch
Rattling noise under the car Catalytic converterPossible, the inside can come loose (ANWB) Oxygen sensorNot mentioned in the sources
Less power Catalytic converterPossible, according to the ANWB Oxygen sensorNot mentioned in the sources
More smoke from the exhaust Catalytic converterPossible, according to the ANWB Oxygen sensorNot mentioned in the sources
Unpleasant smell under the car Catalytic converterPossible, the ANWB mentions unpleasant smells Oxygen sensorNot mentioned in the sources
Higher fuel consumption Catalytic converterNot mentioned in the sources Oxygen sensorPossible, Bosch links good sensors to lower fuel costs

Sources: ANWB, Katalysator (catalytic converter), and Bosch, Lambda sensors: quick and easy testing and replacement. Consulted November 2026.

If something rattles under the car or the exhaust sounds louder than normal, something else may be wrong with the exhaust. You can read more about that in exhaust noise: rumbling, rattling or hissing.

What damages a catalytic converter?

A catalytic converter usually fails because of another problem. According to the ANWB, a broken catalytic converter is often the result of incomplete combustion, for example because of worn spark plugs or piston rings. With normal use and maintenance, a catalytic converter should last at least 10 years or 150,000 km, according to the ANWB.

  • Incomplete combustion: unburnt fuel gets into the catalytic converter and burns there. This can make the converter overheat. See also when to replace spark plugs (in Dutch).
  • Oil or coolant in the exhaust: Walker calls this contamination. Substances that do not belong in the exhaust leave a coating on the inside of the converter. A leaking head gasket or an engine that uses oil can cause this.
  • A knock from below: according to Walker, debris from the road hitting the catalytic converter is the main cause of physical damage to a converter.

That is why we also look at the engine when a catalytic converter is broken. Otherwise you fix the effect, but not the cause.

How do we find out whether it is the converter or the oxygen sensor?

We first read the fault codes and then take measurements. Only when it is clear which part is causing the problem do we advise a repair. This is how it works:

  1. Reading the codes: we read the fault memory and look at which codes are stored and under which conditions they were stored.
  2. Checking live values: with the engine warm, we look at the signals from the sensors before and after the catalytic converter. Bosch advises comparing those values with a measurement of the exhaust gases.
  3. Looking for leaks: we check the exhaust and the intake for leaks before the sensors.
  4. Checking the engine: we look at the ignition, the spark plugs and signs that the engine is using oil or coolant.
  5. Price in advance: we tell you what the cause is and what the repair costs. We only carry out repairs with your approval.
  6. Repair and check: we replace the oxygen sensor or the catalytic converter ourselves, or fix the leak or the engine fault. We then clear the fault codes and check that the message stays away.

More about reading fault codes is on our page about car diagnostics in Rotterdam. If the problem is in the exhaust itself, such as a leak or a rusted-through silencer, our page on exhaust repair and replacement in Rotterdam explains how we deal with it. We do this for all makes, including vans and motorhomes up to 3,500 kg.

Do you have a diesel with a warning about the diesel particulate filter (DPF)? We do not clean, replace or remove diesel particulate filters. We do find the cause of the fault and tell you what is needed.

What does a catalytic converter fault mean for the APK (MOT)?

The APK is the compulsory Dutch vehicle inspection, comparable to the British MOT. If the engine warning light is on, the inspector measures the exhaust gases. According to the APK manual of the RDW (the Dutch vehicle authority), a car can no longer fail on a reading of the on-board diagnostic system (EOBD) alone. For a petrol car, the inspector then measures the CO level and the lambda value, among other things. According to article 5.2.11, the lambda value must be between 0.97 and 1.03.

According to the same article, a petrol car first registered after 31 December 1995 must have a properly working emission control system consisting of a catalytic converter and an oxygen sensor. On the inspection report you may see wording such as “Lambdawaarde wijkt af” (lambda value out of range) or “Emissiebestrijdingssysteem niet compleet” (emission control system not complete). More on this in engine warning light on: will your car fail the APK and in failed the APK on the exhaust.

Frequently asked questions

How soon should I go to a garage if the engine light is on because of the catalytic converter?

As soon as possible, the ANWB advises. That is better for the environment and prevents further damage, because the cause is often in the engine. Call +31 10 479 00 00 with your registration number and we will book in the diagnosis.

How long does a catalytic converter last?

According to the ANWB, a catalytic converter should last at least 10 years or 150,000 km with normal use and maintenance. Walker writes that catalytic converters are designed to last as long as the car. If a catalytic converter fails earlier, there is usually a reason, for example in the engine or a knock from below.

How much does a new catalytic converter cost?

According to the ANWB, a new catalytic converter costs € 500 to € 1,000, depending on the type of car (consulted November 2026). We tell you one price in advance, based on your registration number. We only replace the catalytic converter once it is clear that it is the cause.

How often should an oxygen sensor be checked?

Bosch advises having oxygen sensors checked every 30,000 km. According to Bosch, an oxygen sensor is a wear part that should be replaced when needed.

Does a petrol car need a working catalytic converter for the APK?

Yes. According to article 5.2.11 of the APK manual, a petrol car first registered after 31 December 1995 must have a properly working emission control system with a catalytic converter and an oxygen sensor. The inspector checks this by looking at the system and by measuring the exhaust gases or reading the on-board diagnostic system (EOBD).

Do you also clean or replace diesel particulate filters?

No. We do not clean, replace or remove diesel particulate filters. We do find the cause of the fault and tell you what is needed. For the APK, see the APK particle test for diesels.

Catalytic converter fault? We measure first

Call +31 10 479 00 00 with your registration number. We read the fault codes, find the cause and tell you in advance what the repair costs.

You will find us at Boterdiep 11 in Rotterdam-IJsselmonde, close to Stadion Feijenoord (De Kuip). We work by appointment. For a quick question or check you can walk in without an appointment.

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