Why does my car smell like rotten eggs?

In this series of Q&A posts, we are using google’s autocomplete function to find the most commonly asked questions about cars and giving them each a definitive answer. This series is ongoing and you can find more of our Q&A articles right here.

Why does my car smell like rotten eggs?

The smell of rotten eggs coming from under the hood comes from byproducts and deposits left over from the incomplete combustion of gasoline being burned off when the catalytic converter heats up. This is something that is happening all the time in your vehicle, but there are 2 instances where this might happen all at once and you are able to smell the burn off:

1. You have been driving your car slowly and gingerly for a while, like the good west-coast island-dweller that you are,  and then you put the petal to the metal to pass someone, which heats up the catalytic converter and burns the deposits of sulphur and hydrocarbons that have built up over the last few weeks. If you rarely push the revs when you are driving there will be a larger buildup for when you gun it up the Malahat or pass someone on the highway, which heats up the catalytic converter and it burns off. Pro tip: Once a week drive your car like a teenager.

Or…

2. One sensor on the car that has to do with fuel management is not doings its job, so it is injecting too much fuel and running rich. The catalytic converter is not able to effectively process the byproducts of the exhaust when this happens. When the catalytic converter is overwhelmed from running rich for too long these byproducts are sent out the tailpipe unprocessed and they have a very distinct smell. Another symptom of a bad sensor is that too many by products in the catalytic converter makes it run very hot, burning off all of its buildup, which can cause a very strong odour.

Bottom Line

If the problem comes and goes for a very short period of time after excessive use of the engine at high revs, there is nothing to worry about. If the vehicle has a strong smell of sulphur or other chemicals on a regular basis during normal use, you can assume there is a sensor issue that should be checked out.

It is best not to leave this for your next maintenance visit, as a clogged up converter can get so hot it will literally melt on the inside, then you won’t be able to drive your car over 20kmh because it is so completely jammed up. We can run tests to find the malfunctioning sensor when you bring it in and get your car running right again before you are left with a $1000+ catalytic converter repair bill.

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