Car Owners Hub UK - Built by Drivers, for Drivers

Greetings everyone,

When I park and turn off the key or lock the door, my brand new W204 will sometimes start the exterior lights. It was the taillights that initially started it. After that, I disconnected them until I could figure out what was wrong; however, the following night, I got home to find that my fog lights were still on, so I turned the key to start the car, and they turned off. My sidelights started turning on as soon as I locked the car and turned off the engine. I switched off the lights, made sure the turn signals were off, and then turned off the engine. After that, I gave up and locked the car because only the driver's sidelight remained on. Curiously, when I turn the key to start the car, all the normal functions activate, and any lights that shouldn't be there just go out. Nothing on the dashboard, no intermittent problems.

As I had anticipated, I went for a run yesterday only to find that the battery was dead. The side light came back on and remained on after I left it on charge all day and night. I turned off the side light when I started the car. I went for a spin and came back. I parked the car and noticed that the lights are off. However, I have a feeling they will turn on again soon.

Can you think of anything? It doesn't appear like the light switch is the problem in my case, though I have read that it is in some others. Maybe St. Andrew's?

We would greatly appreciate any assistance you can provide.

Following the advice of the forum, I disconnected the battery for the night and the fault never came back on my Jeep, which was not a Merc (though it was engined at the time).

I noticed it before it caused a flat battery on my W204 because it was always parked in a garage, which I am constantly entering and leaving. The simplest solution was to turn the light switch on and off again. If I had to guess, I'd say it's the SAM module that regulates the lighting. When you turn the lights on and off, you're actually sending a new off signal to the SAM. Since SAM malfunctions were infrequent and it could only activate one side light, I refrained from replacing it.

In addition, I propose a SAM module.

Look for signs of water seepage

Water seepage... or a break in the ground connection could occur.