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Rotten Egg Smell From The Exhaust

A sulphur or rotten-egg smell from the exhaust usually comes from the catalytic converter, often because the engine runs rich or the converter is failing.

Normal risk

Usually not urgent, but have it checked soon. The cause can damage the catalytic converter.

Short answer · Why does my exhaust smell like rotten eggs?

The smell is hydrogen sulphide formed from sulphur in the fuel. A healthy catalytic converter turns it into odourless compounds. A strong smell often means the engine is running rich or the converter is not working properly. Have the fault codes read and the mixture checked.

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Safe checks you can do

  1. Read the fault codesRich mixture or catalyst codes support the cause.
  2. Note when it smellsAfter hard acceleration or with a cold engine is common.
  3. Check for misfiresA rough engine or flashing check engine light adds to the problem.

Questions that narrow it down

  1. Is the check engine light on?Look for P0420, P0172 or misfire codes.
  2. Did it start after refuelling at a new station?Fuel with more sulphur can cause the smell briefly.
  3. Is it a manual gearbox car with an old oil?Old gear oil can also smell of sulphur, though rarely.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Rich mixtureToo much fuel overloads the converter.
  2. Failing catalytic converterNo longer converts the sulphur compounds.
  3. MisfiresUnburnt fuel reaches the converter.
  4. High-sulphur fuelTemporary smell.

Questions about Rotten Egg Smell From The Exhaust

Is the smell harmful?

At normal levels outside the car it is unpleasant rather than dangerous. Do not let exhaust gases enter the cabin.

Will a fuel additive help?

Not usually. Find and fix the rich mixture or converter fault.