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Why does my car smell like rotten eggs?

Ellie Dyer-Brown, 11 months ago

2 min read

iStock-car smell

The most likely cause of a rotten egg smell in your car is a broken catalytic converter, but there are other things that could be at play

Here are five reasons your car might smell like rotten eggs:

  • Broken catalytic converter

  • Battery leak

  • Faulty fuel pressure sensor

  • Worn out fuel filter

  • Old transmission fluid

Now let's take a look at those reasons in more detail and how to fix them.


Broken catalytic converter

The eggy smell originates from the tiny amount of sulphur that's in your car's fuel. If the exhaust system is working effectively this smell is not given out as the catalytic converter transforms it into odourless sulphur dioxide.

However, if the converter breaks or the filters wear down, this conversion process can't take place. As a result, an unpleasant egg smell is released. 

iStock-174798960

The solution: it's likely that a new catalytic converter will be required. Before you do anything else, have the issue diagnosed by a mechanic to make sure this is definitely the cause of the egg smell.

Replacing a catalytic converter costs £381.36 on average according to WhoCanFixMyCar data from 2021.


Battery leak

A dying lead battery will produce hydrogen sulphide gas, which could explain where the egg smell is coming from. This is particularly likely if the battery has been frozen, overcharged, or has suffered a mechanical or electrical defect.

The smell will be strongest when you open the bonnet.

Mechanic inspecting car battery

The solution: either carry out an at-home battery replacement or have a mechanic do it for you. It's important to act fast because leaking battery acid will eat through other plastic and metal components in your engine.

Battery replacement costs £169.70 on average based on 2021 data.


Faulty fuel pressure sensor

The fuel pressure sensor monitors and regulates the flow of fuel into the combustion chamber. When the sensor stops working properly, too much fuel can be injected into the chamber and, as a result, the catalytic converter can become clogged.

As a result, the exhaust byproducts aren't converted efficiently, leaving a rotten egg smell in and around your vehicle.

Fuel injection replacement

The solution: this isn't something that can easily be fixed at home; resolving issues with the fuel pressure sensor requires expert knowledge. Take your car to a garage as soon as you can.


Worn out fuel filter

The function of the fuel filter is pretty self explanatory: it filters dirt and debris from fuel so that these particles don't enter the combustion chamber. A clogged filter will likely cause an influx of sulphur deposits, hence the strong egg smell.

The solution: replace your fuel filter as soon as possible - failure to do so poses a fire hazard.


Old transmission fluid

The gear system in your car is lubricated by transmission fluid. If you haven't flushed the system in a while, it might be time to do so now. Otherwise, the fluid could leak into other systems, causing the smell to intensify (along with causing a whole host of other problems).

Car Transmission image

The solution: you can either flush and replace the fluid yourself, or have a mechanic do it. This job should be relatively quick and affordable.


Should you be worried about an egg smell in your car?

You should always be concerned when you notice an unusual smell in your car because this indicates that something has gone wrong. However, the severity of the problem will only become clear when you have your car checked by a professional.

When it comes to the smell of sulphur, or rotten eggs, this is generally a bad sign. It usually indicates a problem with your catalytic converter; if this component isn't working properly, you and your passengers are at greater risk of breathing in harmful exhaust by-products that would normally be converted into less harmful gases.

For this reason, you should act fast when you notice the smell.


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