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I came back from vacation (20 days) and my 2006 XC90 wouldn't start. The key fob wouldn't unlock which was the first indication and then I just had absolutely no lights so I assumed it was a full drain. I tried jump starting it to no avail so I took the battery in and they confirmed it was drained and the diagnostic report suggested a new battery. I ended up putting the new battery in and after running it for a bit noticed that the fog lights stayed on after I had turned the car off. I tried turning them on and off manually but they would still constantly stay on.

I've disconnected the battery for now to not have it drain again but I'm at a loss. I don't know if this is what caused the original battery to drain or if I shorted something while jumpstarting / removing / installing the new battery (absentmindedly connected the + first instead of the - when installing the new one and had the car alarm go off).

I've checked the fuse and everything seems to be in order, I might check the relay but it seems to just be the fog lights that are staying on and not related components, plus I doubt a relay would weld itself connected, it would make more sense if the lights wouldn't turn on.

Nikola Jankovic
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There is probably a relay just for the fog lights. Its contacts may have fused together. First thing to try is swapping the relay for another know working one.

Often there are identical relays in the fuse box. For example it is likely that the headlight and fog light relays can be swapped for the test

There is probably a fuse for the fog lights which you could pull out so that you don't have to keep disconnecting the battery until it is fixed.

HandyHowie
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Based on our conversation in my other answer, and the wiring diagram you provided - (imgur.com/a/BTP0Vcr), I presume that there is a problem with the module that is labelled 4/56. It receives a CAN BUS instruction to turn the fog lights on/off. If it is not that module at fault, then it must be receiving a valid message to turn the lights on.

It looks like your car doesn't have relays, the lights are switched directly from the CAN BUS module. There are however 2 fuses shown on the wiring diagram that could be removed to stop the fault flattening your battery.

HandyHowie
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