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2004 Volvo V40 - Lights killed the battery

Parked in the garage Wed even, turned car off and closed garage. Thurs morning all back lights were on and battery was dead. What happened and how can it be fixed?

If by “back lights” you mean your brake lights, could be brake light switch failed.

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Tail lights, brake lights, parking lights, every light across the back of the car. . .

If you had looked the front parking lights were probably on also. Charge the battery and make sure you turn the lights off and you may not have a problem.

How can the lights be on if the battery is “dead”? I can understand too weak to run the starter, but dead nothing would work.

How could the lights been on when the car was turned off and locked?

The light switch could do that. Have you charged the battery and used the light switch to see if it turns the lights off. You lock your vehicle when it is the garage ?

The majority of newish vehicles sound a warning tone if the lights are left on. For that matter, my 1970 Ambassador had that feature. This is more likely to be a faulty brake light switch IMHO. And yes, when I parked in an attached garage I locked my car.

My guess is that the brake switch had an issue. Power is usually tied to the switch at all times. For the running lights to be on also there would have to be some sort of bridged wire connection to the brake light circuit. Possibly through a trailer wire connection if yo have one.

It could also be a bad door sensor. Some cars tie the headlights/parking lights into the door sensor so that they shut off automatically when you get out of the car. Even the ones with auto headlights sometimes do that in case you manually turned the switch on instead of leaving it in auto.

If that sensor isn’t detecting that the door opened, and OP is running in any headlight mode other than auto, then it doesn’t know you got out of the car and won’t tell the car to turn the lights off.

I’m going to offer a different possibility, because I had a similar problem with my POS Volvo.
Yes, that was many years ago, but…

When the parking lights, side marker lights, and tail lights stayed on despite the switch being turned-off, it turned out to be a bad ground connection.

The Central Electronic Module may be failing.

Tester

Yes, on a modern Volvo, that is probably more likely than the multiple ground problems that I had with my Volvo.