Does the Sigma immobiliser have a habit of playing up in the cold? Mines (2005) been struggling to unlock/lock the car in the past few days. Today it finally gave up. Tried both keys and both with brand new batteries. Nothing. Zilch. Nada. Keys unlock the car and trigger the central locking (so no mechanical issue). I have never user the keypad.....although only last week I was thinking that I really should get round to setting it up one day.....

However, I found the original key code with purchase receipt I have in the history folder and in 20 years, no owner has ever bother to change it!

So seeing as the keys are probably not the issue (unless they have both died!) is it likely to be an intermittent temp issue or more likely that the receiver is terminal? If so, is it possible to replace just the remote central locking receiver in isolation to the Sigma - or is it a combined unit?

 

With the introduction of the single button keys, the key pcb controls the sigma alarm which then triggers the keyless entry CU and Double lock CU.
If the keys do nothing and the batteries are good, they may need "pairing" to the alarm again.
As you know the PIN for the keypad you can use the teaching instructions in the sigma operators manual.

Relevant wiring diags attached and in case you don't have it, a copy the M30 operators manual

 

Repairing the keys worked a charm.