Symptoms:
• The red light on the key still operates, therefore we know that the batteries are still operational
• When the key button is pressed within the distance of the receiver (in this case, the receiver is located in the rear view mirror but it may also be located on the outside of the door handle) we see either a repeating flashing green or red light, indicating the activation or deactivation of the system immobilizer.
• An alternating green-to-red light indicates that the remote is out of sync and will need to be reprogrammed.
• There is no movement from the door locks at all and no noises apparent from the central locking pump

Steps:
• Test the fuse (in this case Fuse 42 under the bonnet is responsible for the central locking system as well as fuse 3 in the boot section under the mat)
• Ensure we are receiving power at the central locking pump motor through the Red wire and a ground through the Brown wire. We know there is no power issue
• Now remove the central locking pump to test that there are no issues with the vacuum lines which operate the locks. You’ll need access to an vacuum gauge that can output roughly 3 psi. Connect the gauge to the yellow line which feeds into the central locking unit. Operate the locks in both directions and ensure all locks open and close correctly and hold their positions
• If all of the above tests have been successful, then it’s safe to assume that we are looking at a faulty central locking unit.
Repair/Replacement:
• When you have the unit removed from the car, you will be able to take a much closer look at the internal of it and see if there is anything which can be repaired. The unit consists of a motor with vacuum line and a circuit.
• To test the motor simply apply a 12V feed and a ground to either side of the contacts and it should run consistently
• The circuit board was the source of our problems in this example and although the broken joint was visible to us, it was not repairable. However, this is not to say that other boards may not be repairable. Inspect the board under microscope if necessary and if possible attempt to repair. After 20 years, this unit had seen better days so a replacement was sourced which immediately rectified the system and had the central locking running again.

Let us know of your attempts in the comments!