Unusual smells and sounds from your vehicle shouldn’t be ignored, especially a pungent odor like rotten eggs. There are only a handful of reasons why your car will smell like eggs, fortunately this issue is easy to diagnose.


Cracked or Clogged Catalytic Converter

By far the most common reason for this smell is a malfunctioning catalytic converter. The odour is caused by Hydrogen Sulfide, which is a byproduct of combustion. This gas is converted inside the exhaust, to produce sulphur dioxide an odourless (and much less harmful to the environment) gas.


If your catalytic converter begins to malfunction or leak combustion gases, you will begin to smell this mainly at the rear of the car – specifically from the exhaust. (Note: Please don’t inhale your cars exhaust!).

Another symptom is rattling from the converter itself. Inside is a honeycomb like structure which can work loose or disintegrate. If you can remove this from your vehicle, try shaking or hitting with a rubber mallet to see if you can hear movement inside.

You may also see a check engine light on your dashboard, indicating a lack of efficiency from your catalyst. A code reader is a handy tool, which will tell you the corresponding tool – pinpointing the exact


A major reason for your catalytic converter becoming clogged is due to an improperly functioning engine. Excess fuel being introduced into your exhaust can destroy the catalyst inside, resulting in foul smells.

If your engine burns oil, this can cool the catalyst preventing soot from being burned off. Generally these problems can be caused by a wide range of issues from faulty sensors, aging rubber parts or a failing engine – consider consulting a trusted mechanic to inspect your vehicle, or read our troubleshooting guides.


Battery Leaking Acid

A battery being overcharged will produce hydrogen sulphide, causing a rotten egg smell. This is due to the acid inside each cell escaping the casing, as most car batteries are sealed.

It’s easy to identify, as you will see a white crystal substance, typically seen around the terminals of the battery. You may also notice your battery swelling at the sides, due to increased internal pressure.

If your battery is leaking or appears to be swelling, immediately stop using your vehicle and get a replacement is installed.

You shouldn’t touch the battery without wearing proper protection, because the internal acid can cause serious injury to skin, even if it’s dried.


Transmission or Differential Fluid

Petroleum based oils can cause an unpleasant smell once aged or heated outside of the operating temperature range. In older vehicles it’s possible the transmission or differential is slowly leaking fluid, causing a bad odour once up to temperature.

A tell-tale sign of an oil leak is a small puddle in the morning, most commonly the drain/fill plug crush washers – fortunately an easy fix (and excuse to change your fluid!).

Another possibility is a crack in the metal casing causing the leak, carefully inspecting the trail of oil will quickly reveal whether you have a crack in the casing, or simply a leaking crush washer.


In this instance, changing the fluid will cure the odours source. Oil does eventually go bad with age and use, and is easy enough to change with simple tools.

Make sure you inspect the transmission and differential for leaks, to solve the underlying problem of smelling the oil. The leak will most commonly be from the drain and fill plugs, if this does turn out to be the source of the leak your local dealership can provide the correct parts extremely quickly!

A Haynes manual for your vehicle will show you the correct procedure to replace the fill plugs, as well as changing the oil itself. The manuals include pictures and manufacturer torque specs, and are easy to follow.