Had this problem on my 01 boxster. It was the headlight switch. Easy DIY and only ran me 70 bucks or so. It was an Audi part number.

 

6 speed - MAF cleaner DIY - worked like a charm. $7.21 for a can of MAF cleaner. I will note that you do not need to use a screwdriver to pry the plastic bezel piece off.

 

Mine was the switch - in fact, it's happened twice. With the original problem, the dealer replaced the switch - but for a quick solution, swapped it with another car they had in stock (hence a used part). Was fine for about 18 months, but then had the same problem again and I replaced it with a new switch.

It's funny looking back at the earlier posts where I was saying "If it's the switch, surely the lights wouldn't stay on?". Funnily enough, one night - at speed, on an unlit road - they didn't. That was interesting!

 

Used "lectro-motive"by CRC electronics contact spray cleaner through the center of the switch by removing the ****, and have had no problems since. Cheap and easy.

 

^ much easier than taking every thing apart, and I wouldn't have scuffed up the plastic bezel wring.

 

It was the headlight switch on mine. Replaced the switch and it has worked fine since.

 

I just had a similar issue pop up on my 2002 C4S.

I turned the headlights on and almost immediately the ABS/PSM failure lights turned on. The super odd thing too was when I went to turn off the car, it kept running (ex. I turned the key to “off” and pulled it out of the ignition, and nothing happened...)

The car DID turn off though after I turned the headlights off.

can anyone confirm the connection between ABS/PSM + headlamp switch + ignition?

I feel like this is a Click and Clack kind of question...