Smoke and smells
Rotten egg smell from the car: what it means
A rotten egg or sulphur smell from the exhaust usually comes from the catalytic converter when the mixture is too rich or the converter no longer works well. If the smell comes from under the hood rather than the exhaust, the battery may be overcharging, which is more delicate.
Check without hurrySmoke and smells
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. If it comes from the exhaust and the car drives well, have it checked without hurry, but do not ignore it. If it comes from the battery, the battery looks swollen or the charging light is on, do not touch it and get to a garage as soon as possible: there may be corrosive and flammable gases.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Rich mixture: excess fuel overloads the catalytic converter and smells of sulphur.
- Worn or damaged catalytic converter: often after misfires or oil burning.
- Fuel with more sulphur: sometimes the smell changes with a fill-up and disappears.
- Overcharged battery: a faulty charging regulator makes the battery give off gas.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What is a catalytic converter and why does it fail? What is an oxygen sensor and when does it fail?
Car system: Exhaust and emissions
What you can check yourself
Notice whether the smell comes from the exhaust or from under the hood.
Check whether the engine light or the battery light comes on.
Look at the battery without touching it: it should not be swollen, wet or smell strongly.
If the smell goes away after filling up at another station, it was probably the fuel.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0420Catalyst system (bank 1): efficiency below threshold
- P0430Catalyst system (bank 2): efficiency below threshold
- P0172Mixture too rich (bank 1)
- P0300Random or multiple cylinder misfire detected
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: fuel trims, oxygen sensors before and after the catalytic converter and system voltage with the engine running.
Frequently asked questions
Is the rotten egg smell dangerous?
From the exhaust it is usually a nuisance and a warning about the mixture or the converter. From the battery it is: step away and ventilate.
Does the catalytic converter need replacing?
Not always: if the cause is a rich mixture and it is fixed in time, the converter can recover. If it is already damaged, code P0420 usually confirms it.
Can the air conditioning cause it?
A damp or musty smell when switching on the air conditioning comes from the evaporator, not the exhaust, and smells different.
Keep reading
- Exhaust and emissions: how they work and what fails
- Engine warning light on and steady: what it means and what to do
- Smell of gasoline or diesel in the car: what to do
- Car uses more fuel than normal: causes and what to check
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means