Have a 2004 Toyota Corolla with about 110,000 miles. When I ride the car with the heat on, every now and then it smells like rotten eggs. I looked online and people say I may have a bad catalytic converter but I'm not entirely sure I do since the problem isn't very frequent.


  • Are you sure it only happens when the heat is on? How long has this been happening? That sulfur smell is a common and pretty clear symptom of a bad catalytic converter. But I'm at a loss as to why that would be tied to the heat in your car.
    – cdunn
    Jan 9, 2016 at 0:36

2 Answers 2


Has Service bulletin EG028-04 been performed? This TSB reprograms the ECU so the fuel mixture is set closer to Stoichiometric and slightly to the lean side. The TSB suggests a new catalyst, We have done lots of these without replacing the catalyst.

The rotten egg smell from the exhaust is caused by the catalyst releasing hydrogen sulfide gas. This happens when the catalyst is at operating temp and is fed a rich mixture. As is the case when the engine is loaded.

Catalysts absorb sulfur when fuel mixture is lean and release it when the mixture is rich. It is not a sign of catalyst failure but one of normal operation. Low sulfur fuel was mandated partly to reduce the amount of hydrogen sulfide in the atmosphere since is a major cause of acid rain. Excess Sulfur content is one of the many things that constitute an "off spec" batch of fuel that would end up at a low price fuel retailer.

I can think of no reason this would only happen when the heater is on. But this does mean the exhaust system should be carefully checked for leaks.


If the smell is from the internal heating system and not from the exhaust system then this is likely to be due to organism (mould, bacteria, ...) growth in the air distribution ducts and heater. This is a well known problem. Proprietary products are available that are intended to cure this problem and adds are some standard household products will work in many cases.

I'd personally try a spray based on "Benzalkonium Chloride" (or one of the many name-variants. (Quaternary Ammonium Salts, Lauryl Dimethyl Benzyl, .... .... ....).

This is used in the majority of home disinfectants, and in many cleaners, moss & mould removers, carpet shampoos and more, at typically around 0.1% concentration. Using it in the same manner as mentioned below has a good chance of working.

HERE is a typical discussion with commercial products being suggested. They note

  • You can do an anti bacterial treatment for your A/C by yourself. There are special antibacterial cleaners that you have to spray on the air filter once a month. You can also try cleaning your air conditioner at least once in a month to ensure that it produces clean and filtered air. Try products like Klima-Cleaner's. This product removes odors from the car and removes water condensation on the air conditioner system too. This can cost about $15 per bottle and can last you a long time. Other products you can try include GardX One Shot Deodoriser, and Kleen Air-Air Conditioning Cleaner & Purifier. Both of which cost about $45-$100.

[HERE(https://www.youtube.com/watch?v=CzYlp9dm7AI) is a Youtube advert for a "3 minute" fix. Efficacy and cost unknown to me.

Just found this - SE 2011 on the same subject .


Custom stuff

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