405 Immobilised!

[carlo14]

Was wondering if anyone here has a good knowledge of Peugeot keypad immobilisor systems fitted between 1992 - 1996, especially whether they can be removed completely (as opposed to disconnected).
 
I've noticed on a few occasions, which have became more frequent, my 405 petrol automatic wouldn't start unless you locked it with the plip, then unlocked it then tried again.  Sometimes this needed two or more tries.  (I have bypassed/disconnected the keypad immobilisor without any issues, like I do on all my 405s so they start without needing to put any code in.)
 
Wednesday morning the car was flat as a pancake, I eventually jump started it and left it running for an hour, although the battery light was still on suggesting a battery/alternator issue.  This morning it's back to it's old tricks, won't start at all despite numerous attempts at remote locking/unlocking.  I've even re-connected the keypad which seems to work i.e. putting code in turns it from red to green but the car won't start, and sometimes the keypad peeps for 30 seconds on red.  Looks to me like the battery draining has had a catastrophic effect on the immobilisor, which was showing signs of playing up anyway.
 
Spent a fair bit of money on this car too, so would like to keep it without spending a fortune, as I expect not many garages will be in tune with 25 year old French immobilisor systems.

Many thanks in anticipation.

[xantia_v6]

Try disconnecting the battery, wait a couple of minutes then reconnect. The ecu may not have reset cleanly on a jump start.

[white exec]

Try also an Alarm Reset, which works on similar vintage of XM:
- disconnect battery for 60secs+
- key removed from IGN
- close all doors, boot
- reconnect battery, and within a few seconds lock and unlock the car with the Plip
(locking may need two presses of the Plip lock button - make sure the knobs/whatever go down)
Alarm etc is then reset.

Red+Green on keypad normally indicates a keypad/immobiliser fault.
Red+beeps = incorrect code
Green should mean ok to start. If it doesn't fault may be elsewhere.

[Oldpug]

Long time since I played with one of these,all I can suggest. Is there a small square button on the steering cowling?
If so press and hold the button and switch the ignition on.The four ways should flash for a few secs,immobilizer is then reset.

[carlo14]

Thanks guys. I'm starting to believe the battery's cooked which could be causing all the probs, will replace that first and take from there.