New catalytic converter smells; what’s normal?

(Last Updated On: April 11, 2023)

A catalytic converter takes harmful pollutants and converts them through a chemical reaction into less harmful water vapor and carbon dioxide. Once a catalytic converter has been replaced, you wouldn’t expect unusual smells to come from it, so what do new catalytic converters smell like, and when is it not normal?

Normal new catalytic converter smells

Catalytic converters have a couple of usually expected smells for the first day or so. All unusual smells should disappear after an hour of driving, but some can stick around for a few days.

  • Label burning – Catalytic converters have labels applied with supplier part numbers attached; they are usually not removed by mechanics before installation, even though they should. The labels burn away as the catalytic converter heats for the first time. As well as a burning smell, you may notice a small amount of smoke, but this will only last a couple of days maximum.
  • Exhaust putty – When a new section of the exhaust is replaced, the catalytic converter is included. A sealing putty is applied around the exhaust gaskets to aid sealing. Depending on the sealant/ exhaust paste used, some can take up to 24 hours to cure and smell funny for a little while after.
  • Oil coating – Catalytic converters and all new exhaust components have an oil coating to protect them from rusting when stored on the shelves as new. The exhaust coating usually burns off within a few hours.

Bad new catalytic converter smells

  • Strong exhaust fumes smell – Pin hole leaks between the welds or joins that haven’t sealed, will allow a small amount of exhaust gasses to escape. You would expect a louder noise from the exhaust but with minor leaks this isn’t always the case. There will be a stronger smell of exhaust gasses in the cabin because the exhaust fumes are not pushed out of the back of the car like normal.
  • Sulphuric smells – Catalytic converters convert the small amount of sulfur found in fuel into odorless sulfur dioxide. When the catalytic converter has failed, you get a strong eggy smell: the smell of sulfur is no longer being converted. When a new catalytic converter smells like rotten eggs, there is no doubt it is bad.

What to do if you find the new catalytic converter smells?

If your new catalytic converter smells remain after 24 hours, you should return your car to the mechanic that fitted it for inspection. There may be no problem, the catalytic converter is still ‘bedding’ in, and there is nothing to worry about. A mechanic will inspect the smells and even plug the car into an OBD code reader to check the operation of the catalytic converter and o2 sensors. Without an OBD reader having the emission levels checked by an exhaust gas emissions tester will determine if the catalytic converter is bad.

However, it is possible that the catalytic converter is or has gone faulty, most vehicle components come with a 12-month/12,000 miles industry standard warranty, but certain garages offer a more extended warranty on catalytic converters. As you would expect, it would usually be replaced under warranty, providing no further issues are causing the new catalytic converter to fail.

Other signs a new catalytic converter has failed

  • Changes color – If the catalytic converter is glowing red and its exterior turns blue when it has cooled down, the temperature inside the catalytic converter is far exceeding its operating temperatures. This happens because the engine is either running lean or rich; there are lambda (o2) sensors on either side of the catalytic converter, which have an input on the air-fuel mixture ratios. Although when o2 sensors are faulty, it usually triggers the engine management light. To be clear, if the catalytic converter color changes, the catalyst will have irreversible damage.
  • Loud noises – The catalytic converter is no different from any other exhaust section; if the internals fail, they will either rattle or have an unusual raspy tone when revving the engine.
new catalytic converter

What would cause a new catalytic converter to fail?

Catalytic converters usually fail because of an imbalance in the air-fuel mixture combusting in the engine. When this happens in a new catalytic converter, it could indicate a problem with one of the o2 sensors.

A catalytic converter can also just be supplied faulty; as you can imagine, aftermarket exhaust components are made in huge batches, so you can expect one or two to be provided faulty. It will just be bad luck, as unless the catalyst is rattling, a mechanic can’t tell that it is defective before installing it, and then you run it for a few days.

Do you have to break in a new catalytic converter?

New catalytic converters have a bedding in process when new; they need to get up to temperature to burn away any contaminants without getting too hot, so keep the revs down for the first 15 minutes. Then they require cooling down naturally, so idling for a few minutes and then switched off. Immediately starting the car and thrashing it down the road, you risk causing a blockage of the catalyst or causing the internals to fail.

Most mechanics usually break it in for you after replacing the catalytic converter. When installed, they will start it and leave the idling while doing their final checks, ensuring no exhaust leaks. The 10-15 minutes they have the car idling on a ramp is enough to break in the new catalytic converter.


New catalytic converter smells are usual and should disappear after a day or two. If any unusual eggy smell or gas fumes smell, you should take the car back to the mechanic to inspect. Occasionally, aftermarket catalytic converters are installed faulty but not at the mechanic’s fault because it is impossible to tell beforehand. A newly replaced catalytic converter should have some form of warranty if the part is faulty. However, it would be best if you were sure there are no further problems with any other factors of your car that can affect the catalytic converter.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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