The car is a 2006 Subaru Outback with 82,000 miles. At low speeds we sometimes get a strong smell like natural gas in the cabin. Took it to the dealer twice, but it behaved -- no smell.

The car has mostly been driven short distances around town, with occasional longer trips. More recently, in the past 18 months, it's gone to New York City 7 times, round trip distance about 1500 miles.

Three weeks ago it had its head gaskets replaced. Also fluid replaced in the front and rear differentials.

After that we started noticing the smell. It's not there all the time; when we first start the car there's no smell, then after a bit there's a burst of odor that persists for a while, and comes and goes during low speed driving. Our last trip to New York was after this started; no smell at highway speeds. But after the trip the smell was still there at low speeds. Just not when the service folks are looking at it.

I just got it back from the dealer after its second visit. They kept it all day, and had four people drive it around town. Nobody smelled anything, except the service manager who mentioned that once he got a slight whiff of natural gas, but he decided that was from outside, since there's nothing in the car that would smell like that. But that's exactly what I smelled. (and, no, it's not hydrogen sulfide; I know what that smells like).

So I'm appealing to the wisdom of the Internet: does anybody have any idea what might be causing this?

  • Welcome to Motor Vehicle Maintenance & Repair! Commented Dec 11, 2018 at 22:51
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    My suggestion is something has crawled into the car and has died ... not sure that would smell like natural gas, but it's about the only thing I can think of which would start stinking in small bursts. You might try removing the seats and cleaning the carpet. Pull back the carpet in any loose places to see if you can find anything. If all else fails get an ozone generator and let it flog the car for an hour or so. Commented Dec 13, 2018 at 23:58
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    Check the cabin filters if it has them, if they get damp or wet, mold will grow on them.
    – Moab
    Commented Dec 14, 2018 at 22:18
  • I actually had a half eaten rabbit head inside my engine bay. I even have a photo. It smelled only sometimes. The second thing it could be, is water leaking inside your car. Check your boot and passenger seats for water. Once you get inside the car, the air could circulate around. Last, but not least, it also might be your "natural gases" 😁
    – h00ki
    Commented Jan 9, 2021 at 0:26
  • Too late for this question but failed catalytic converters can smell like rotten egg . Just to add that into the mix
    – ajayel
    Commented Feb 9, 2021 at 8:49

2 Answers 2


Being that the exhaust manifolds have been off to do HG, have they replaced with a new gasket? If it leaks from there, the exhaust smell will filter back to the cabin at slow speeds (diluted at high speeds). See igf you can smell it under bonnet near the manifolds


You mentioned the diff fluid was replaced. It could be that diff fluid wasnt cleaned all the way off when it was replaced. Completely normal, If you can just power wash the differentials and see if that works.

Three days ago I changed my transmission fluid and I'm still smelling it (although its going away) even though we didn't spill much.

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