I was inspecting a used car and the oil and the transmission dipstick smelled of exhaust (carbon monoxide?). What does this mean? Does it mean that the engine's gone bad and I should stay away from that car?

  • 28
    carbon monoxide is odorless. Commented Oct 16, 2019 at 23:47
  • 6
    It means you should bring along someone who knows something about cars. If I was selling the car then I would know I can take you for a ride once you tell me that the tires smell rubbery.
    – MonkeyZeus
    Commented Oct 17, 2019 at 18:25
  • 5
    ...you were smelling the dipsticks???
    – Z4-tier
    Commented Oct 18, 2019 at 4:22
  • 1
    I don't suppose you went around and smelled exhaust either? Does it smell like new car exhaust or old car exhaust? Diesel exhaust and gasoline exhaust also smells different, and even more so if they came from engines that were not working properly.
    – Nelson
    Commented Oct 18, 2019 at 10:14
  • If you smell an odorless gas, it is probably carbon monoxide. Commented Oct 19, 2019 at 2:36

3 Answers 3


First of all you're not smelling CO (carbon monoxide) as that is an odorless and colorless gas. What you are likely smelling are combustion byproducts and that is not at all uncommon in the engine oil. The transmission oil is another story and I suspect you are just smelling the oil itself.

The thing I'd be concerned about is a sharp "burnt" smell that indicates overheating or other heat-related issues.


Smelling AT fluid is (or did ) give a good hint of the transmission general condition , but it requires experience. The clutches and bands have material like brake pads and if they slip, they heat very quickly although running in oil , and have a "burned" smell.

  • Well, it comes from experience of working on cars and knowing what causes what when you have particular smells. It probably isn't going to work for the OP though since they thought they were smelling Carbon Monoxide.
    – Nelson
    Commented Oct 18, 2019 at 10:13

If the motor oil has a distinct smell of exhaust fumes, it might also mean that the cylinder head gasket has become leaky. Transmission fluid should never smell of exhaust gases, but can get a certain burnt smell from the clutch pads.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .