It's a long time since I asked a question on here,as since getting some really helpful answers 5 years ago,all has been well with my Discovery TD5 (year 2,000) Millenium Edition.
So,to today's problem! Used the vehicle this morning and on arriving home,locked it with the fob in the usual manner. Went to pick up the 'other half' this evening and on pressing the unlock button,no comforting 2 'clonks' were heard as the locks opened. The interior lights came on but nothing from the locks. On pressing the fob lock button,the interior lights dimmed and then went out as normal while the red dash light rapidly flashed for about 20 secs before going out and flashing at the normal slow rate-again as normal. Checked the above several times and the result was the same.
Then I tried using the key in the lock. The door unlocked and upon opening the alarm triggered with the red dash light rapidly flashing. On pressing the fob open button,the alarm stopped but the dash light rapidly flashes and remains like that until the engine is started. The engine starts normally and runs fine,except that in the lower left of the mileometer display,there is a strange flashing symbol which I've never seen before. It's like an 8 but with a diagonal bar running across the 8.
If the key is used to unlock the door and the door is just opened,after pressing the fob button,the alarm stops but the red dash light keeps rapidly flashing and doesn't stop until the fob lock button is pressed,when it extuinguishes after about 20 secs and the interior light goes out. However the sill button doesn't activate and the door a unlocked until locked with the key.
Whilst inside the car,the dash interior master locking switch which normally opens all locks or closes all locks whilst within the vehicle,remains inoperative-I cannot open the passenger door or any of the rear doors. This is a right bummer as I cannot get anything much in or out of the vehicle!
I'm extremely puzzled as to why the main locks refuse to budge,even with the interior master switch. I've not tried the emergency key access procedure using the door key to enter the 4 digit number,as reading through the manual,this only seems to be relevant if the key fob has a problem and engine mobilisation is needed. The engine starts fine and the vehicle drives normally with the key in position.
I'd appreciate any hints/tips/advice that members can offer,especially relating to the very strange flashing symbol in the bottom left corner of the LCD mileage display-the handbook does not refer to this at all and has not been a feature of the vehicle up to now.
Got to fix it as I'm needing to load all my toolboxes in 2 days time to go out on a job!
Cheers-Adrian
PS. Hope this ends up in the right place as I couldn't find anywhere else to put it!

Not that familiar with Discos but have you been through the basics and checked all the fuses, there is probably one for the central locking in there somewhere.

2002 wiring diagram here if that helps http://www.landroverweb.com/Pdf-files/Defender MY2002 Wiring Diagram.pdf Don't forget fuse links in engine compartment

Many thanks for your suggestions and the attached circuit diagram,which I found most illuminating! Today ran through all the usual simple checks,fuses etc-all checked ok. From some leads I got off this site,I then checked the BCU for corrosion/water ingress-all was good. Next on the list as the prime no 1 suspect was the fusebox with integral IDM. Inside I found some water together with a small amount of corrosion. Removed unit from vehicle and separated the electronics from the plastic enclosure. This afternoon was spent drying the unit and cleaning off the corrosion,before treating with a special pcb lacquer. Tomorrow we shall put back into my Disco and see whether we've been successful in fixing the problem or whether we shall need to source a replacement-fingers crossed!
I'll let you know how successful,or otherwise,I've been!
Cheers-Adrian

Had the corrosion eaten through any of the pcb tracks?

No,the pcb tracks were good. The corrosion was around the base of 3 of the big flat pins on one of the brown connectors at one end of the unit and water was present across some of the tracks in the vicinity-bottom right attached pic. Hopefully I've caught things early and my affords will be rewarded! The vehicle has seen a lot of water dumped on it over the last couple of weeks thanks to our glorious climate and obviously some water has leaked in from somewhere!
Cheers-Adrian

Eureka! Fitted the fusebox back this morning and reconnected the battery. All appeared well,so inserted the key and turned it with fingers crossed. Turned a bit more and the engine fired up along with all the door locks clonking as they unlocked-big relief at that one! Checking all the circuits one by one,we established all was good. Everything is now back to normal and we don't have to get a replacement fusebox,although I had one sourced on E-Bay just in case.
This site has been invaluable in following up leads and finding out more knowledge of the systems involved from those who have pioneered and gone before.
Cheers-Adrian