Repeated tailgate open warnings while driving and alarm triggering while parked.

[TheJase]

Hello UKPassats. First post here.

I have a 64-plate B7 Passat Estate 2.0 TDI Bluemotion which has been running pretty reliably since I got it about 4.5 years ago. About four weeks ago, the original battery seemed to have reached the end of its life and failed to start the car one frosty morning. Metering it showed it was only providing 10V. I ordered a Bosch S5 replacement and fitted it with no problems and the car has been running just fine since, until this last Saturday (the day after Storm Eunice), when it developed a fault about 30 minutes into a 155 mile journey.

The dash started sounding a warning chime while the MFD would indicate that the tailgate was open and the interior lights would come on briefly. I check the boot and it was firmly closed. This would repeat a few seconds later, getting increasingly frequent. This continued for most of the journey, with an occasional pause for a few minutes, and the same thing happened on the homeward journey later that day.

I searched the web and found several reports of a door open sensor in the tailgate locking mechanism going faulty, sometimes due to washer fluid leaking from a split hose.

Sunday morning, I took the car out again for a short journey and parked it up. When I returned to the car, I found the alarm had been going off and the rear fog light was lit. Once I got home and parked up, the fog light lit up again when I turned off the ignition and remained lit after I locked the car. I removed the trim from the inside of the tailgate and inspected the locking mechanism, but could find no evidence of moisture getting to it. I ordered a replacement lock from eBay anyway. When I took the car out again that evening, the fog light was still lighting up after turning off the ignition and remaining on after locking the car.

My wife tried to use the car on Tuesday morning, but found the (new) battery had drained. It was only providing 9V. Being drained so low, I thought that it would require a very long drive to get it charged up from the alternator after a jump start, so decided to buy a charger to give it an overnight charge. During the day, the alarm kept going off repeatedly despite the very flat battery and I could find no way to lock the car with the alarm disabled, so I ended up leaving it unlocked. I also tried disconnecting the cable from the potentially faulty tailgate lock, but this made no difference, the alarm still went off occasionally.

Wednesday afternoon, the charger arrived and I disconnected the battery from the car and put it on charge for the rest of the afternoon and overnight. Yesterday morning, it still hadn't finished charging but wasn't far off as it was showing 13V on my meter. I put the battery back in the car with no problems, fitted the new tailgate lock and connected it up, then drove around 20 miles to work, parked up for a few hours, then drove home. No alarm or fog light while it was parked, and no chimes or MFD warnings of the boot being open while driving.

Then, yesterday evening, the alarm went off again after the car had been parked on my driveway for a hour. When I got to the car, the fog light was on again. After unlocking and locking the car to silence the alarm, the fog light went off about a second after the doors locked. Unlocking the car made the light come on again, then locking it made it go off a second or two later. Then the alarm went off again. I ended up leaving the car unlocked again all night to avoid annoying my neighbours.

So, what initially appeared to be a fault with the tailgate lock hasn't completely cleared up even after replacing it.

Before I get the car booked in for a potentially time consuming and expensive electrical diagnostic, can anyone advise me on other things I can check myself?

Thanks for any advice.

J