I have struggled to keep the alarm / remote door locking on my 97 VW gof working for many years. Often I have had to adjust the ceramic trimmer on one or other of the two fobs to correct for frequency drift. Just recently one of the fobs failed to work completely. I find it is useful to check the ferquency of the fobs by operating it next to the aerial of an old Faairmate HP-100E vhf scanner that I have. With experience I have found that its frequency is around 417.975 MHz. The good fob was transmitting but there was no signal from the non-functioing one. On rotation of the trimmer I found that at some critical point it would suddenly transmit and then stop. I suspected that the trimmer was so old, it was exhibiting an internal short. I decided to replace it and I am pleased to say that that has restored it to working order. Worth trying this if you have a non working fob as they are now unobtainable.

Although the pcb is mostly surface mount components the trimmer is easily replaced as it has plated thro holes. The trimmer is 0-3pF and needs to be 5mm in diameter. Not easy to source , but I found an American radio ham supplier who could supply Tusconix 0528-012A1 0-3pF in packs of 3 for 3$ with post charge of 15$ to the UK.