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.

  • Welcome to Motor Vehicle Maintenance & Repair! Is the question just, "What's going on with the fog lights?" or is there something else here. You don't really pinpoint a question. Sep 7, 2018 at 11:53

2 Answers 2


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.


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.

  • I would test the relay with a multimeter and a battery first. It isn't rocket science, and usually you don't have another known working relay laying around unless you're a mechanic working for a garage. For most people, having another known working relay means you have to buy the working relay, and it costs some amount of money that will be wasted if the relay wasn't at fault.
    – juhist
    Sep 7, 2018 at 13:17
  • 1
    @juhist 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.
    – HandyHowie
    Sep 7, 2018 at 13:39
  • @HandyHowie I don't believe it's the relay, the lights turned off when the car was with my mechanic and then turned back on when the car was put to park outside. The chances of the relays being welded in the first place is possible but having it happen multiple times either doesn't seem likely or is indicative of a worse problem. I'm starting to think that something chewed up a wire because of the intermittent nature of the issue, but if that was the case wouldn't it cause the lights to not come on, and not the opposite? Sep 10, 2018 at 23:16
  • I looked through the wiring diagram (imgur.com/a/BTP0Vcr) but couldn't make sense of it. The fog light button seems to function - an indicator light turns on when pressed and off when pressed again but the fog light remains on regardless. Sep 10, 2018 at 23:20
  • Is it both front and back lights that are coming on?
    – HandyHowie
    Sep 11, 2018 at 4:46

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