Trouble-Shooting the remote control

On occasions, the remote control does not work as you expect.  Here are the most common symptoms:

1. Remote does not unlock the car after a week away:

The central timer inside the car switches off the remote locking module after 96 hours to conserve the battery.  The doors can only be unlocked with the key - but the programming is still in place.  The key-fob will work normally after the car engine has been run for a while.  Do not try to reprogram the fob - it is not needed and the RC module is shut down, so you won't get a tone.

2. Key Fob will not work, even with new batteries - programming chime does not sound in car.

The PCP inside the fob can get very dirty and dusty, and may prevent the contacts from being closed.  Open the Key-fob again, take out the batteries, gently push the buttons through and the PCB will lift out together with the rubber membrane.

Turn the PCB over and you will see the contacts on the circuit board:

Clean these contacts.  One owner recommends a tiny amount of isopropyl-alcohol (tape head cleaner) while another recommends gently rubbing with a pencil erasure.  Clean both sides of both batteries to ensure a good contact between them.

3. The PCB contacts have been cleaned, but the fob still doesn't work:

Sometimes the little contact pads beneath the membrane buttons can wear away.  One owner has repaired his using metal paint, another has stuck tiny pieces of tinfoil onto the round contact patch, while another has rubbed the contact pads with a graphite pencil.  Anything that will bridge the contacts will do, as long as it doesn't come adrift and cause an unplanned short somewhere.

The membrane buttons of the fob are a small technological wonder.
Whereas their outside is made of a soft molded rubber inlay with a nice tactile feel the insides, where the buttons are supposed to touch the exposed copper pad contact areas on the PCB, these are made of conductive rubber.
 
Their contact pads bridge the meshed E-shaped copper areas to provide power to flow to the encrypting/decoding IC (integrated circuit) which in turn switches on and modulates the transmitter/receiver, that's the encapsulated silver 1” x 0.5” coloured object at the bottom of the PCB.

The latter sends an encrypted radio signal to the car. When that code is matched with the previously received code kept in the car alarm system memory the car responds by locking or unlocking the doors and enabling/disabling the alarm circuit. At the same time a new code is set by the cars alarm system and transmitted back to the fob for the next opening/closing event!

This is why the fob ‘loses its memory’ when you replace the batteries: it forgets the previous code received and you have to therefore ‘re-program’ i.e. synchronise the car and remote fob.

So – the conductive rubber contacts carry the total operating current for the fob each time you press a button -- and current as well as mechanical friction is what is wearing them out after a few years.

If your fob does not work anymore, or is losing operating distance, it could be of course weak batteries but also contact wear, increasing electrical resistance. The conductive rubber dots become non-conductive and less (or in the end no) power flows to the circuits.

One way to repair this I have found is replace the worn conductive dots with a conductive coating.
Here’s how.

Prise the fob apart ending up with 4 parts as shown on the photo (not counting the 2 batteries).
Get some ‘electrically conductive coating’ e.g. Bison or other brands. It's expensive, as it contains pure silver; around £12 in Holland.
Shake well and use the brush to paint the stuff on as described in the user instruction, usually 2 -3 coats, drying for 12 hours or so between coats.
Put the batteries in the right way round, click the unit together and reprogram. When you have more than one fob you must program those together in one single session! (see Programming the Remote).

4. Still no joy - fob still doesn't work.

Check that each of the batteries show the same voltage, approx 3V.  If one is much weaker it will significantly reduce the PD to the circuit board.  Always change both batteries - Lithium CR 2016

Thanks to Ray Pestman-Steyn and EricR