The most likely cause of a rotten egg smell in your car is a broken catalytic converter, but there are other issues that could be at play.
Book a car repair nowEllie Dyer-Brown
Tuesday September 24 2024
5 min read
The smell of sulphur - or rotten eggs - is obviously unpleasant, but more than that, it's a sign that something is wrong with your car. Find out what it might be and how to fix it in this guide.
The main culprits for the horrible smell of rotten eggs are the battery and catalytic converter. If a battery is overcharging or breaking down, it produces hydrogen sulphide, which smells like eggs. Catalytic converters can sometimes have a similar smell when they break or overheat.
Matt Wrankmore
Head of FixMyCar's Garage Network
The most common cause of a rotten egg smell is the catalytic converter, which transforms harmful pollutants into less toxic gases, like nitrogen and oxygen.
When hydrogen sulfide passes through the catalytic converter, it is usually turned into water or steam. But when the converter is clogged, it changes into sulfur dioxide, which is where the rotten egg smell comes from.
Find out more about car exhaust systems here.
A new catalytic converter is probably needed to fix this problem. Before you do anything else, have the issue diagnosed by a mechanic.
Replacing a catalytic converter costs £433.80 on average when you book through FixMyCar.
Car batteries contain sulfuric acid, which usually stays inside the battery case without causing problems. However, if the battery is damaged, the acid can leak out, causing a strong smell like rotten eggs that is strongest when you open the bonnet.
This guide explains how to test whether your battery is in good working order.
You can 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 £225.93 on average, based on our data.
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 discussed earlier, faulty catalytic converters often produce a rotten egg smell.
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.
The average price of a fuel pressure sensor replacement is £99.41, according to our data.
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 to the catalytic converter, hence the strong egg smell.
Fuel filters need replacing approximately every 20,000 to 150,000 miles; the interval varies between makes and models. This job is often done as part of a full service.
You can expect to pay between £60-90 for a filter replacement.
Older manual cars often use a sulphur-based lubricant. If it starts to leak and comes into contact with a hot engine component, you may notice a rotten egg smell.
Leaks are more likely to occur when the transmission fluid is old; you should replace it every 30,000 to 60,000 miles. This guide explains how to check your car's fluid levels.
You can either flush and replace the fluid yourself or have a mechanic do it. This job should be relatively quick and affordable.
A transmission fluid replacement will typically cost between £150-200.
You should always be concerned when you notice an unusual smell in your car because it indicates something is wrong. However, that doesn't mean you should panic. The severity of the problem will only become apparent when you have your vehicle checked by a professional.
If you have any other issues with your car that you'd like to try to diagnose yourself, check out our guide on self-diagnosing car problems. The more you know about how your car works, the better.
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