My 2001 S-Type 3.0, Battery died and wouldn't take a charge past 62%.
placed back into car and drove for 2 days without any problem.
Then it wouldn't start (no crank from the starter at all) Couldn't jump start...same. Replaced battery (less than 1 yr. old, under warranty)
Started and drove that night and noticed the airbag warning light was lite very dimly when I turned it off. Next morning for @ 25 miles, all is well, till I parked and 2 hours later, same thing, won't start. Called AAA and they came and said battery is fine, it's a starter problem, tapped on the starter and it started. Drove home and the car Wouldn't Turn Off? Pulled the key out and it still is running. I disconnected the battery and turned the key on and tried to start it and it turned off. Now the airbag light is on only when the engine is off, and its very dim? and it won't start again? I tapped on the starter and nothing? Any help. I searched the forum and can't find anything like this.

 

I'd be checking for a bad earth connection first... In the boot (trunk), alternator and starter first...

 

Check power, grounds, that trunk is dry (remove spare) and all fuses and relays.

Suspect ANY add-on electrics.

 

Thank you, I'll try that.

 

Thanks JagV8, The trunk has been taking on water the last couple of years. I guess the gaskets are getting old (although they look fine) the trunk has some water in it. I didn't get a chance to dry it out yet. Makes sense. I'll do that 1st. It takes water while parked for the 3-4 months of winter. Bad winter both this and last.
Thanks Again.

 

There are modules in there and they connect to the whole car computer networks thus causing all manner of faults.

 

Any idea how the water is entering the trunk, it has been parked for 4 months, only rain and snow.
Do you think the trunk gaskets, they look good.
Also, there is a small black hose @ 2-3' long, not hooked up?
Also there is a small USB type connector coming off near the battery not connected and I cannot find a receptacle?

I have a photo, but I don't know how to upload it.

 

Check the seals around the tail lights...