I own a Ford Focus 1600 tdci eco on a 2012 plate. 46,000 on clock. Keyless start. Serviced last year. No problems until now. Vehicle was left standing for 3 weeks whilst on holiday. 
Fob opens and closes doors, all electrics work, lights etc. However the vehicle will not start. Error on dash Immobiliser fault service required. I have had a key expert with his diagnostic equipment test and re programme another key to no avail. I have disconnected the battery and left for 25 mins still the same. The battery is working at 70 percent Ish. Vehicle towed to a Ford garage and they are unsure of the problem. Any advice would be greatly appreciated.
Many thanks
paul

Have u tried entering with the spare key? 

If you haven't changed the key fob battery it might be worth trying a new good quality one. It might be a cr2032 type. A low battery might still open the doors but might cause problems starting. They should be changed every 5 years.

I have tried everything. The Dealership thinks it could be the PCM or BCM. I have researched both and it doesn’t appear to match the symptoms. As stated there was nothing wrong with the vehicle before I left it standing for 3 weeks. To me , a complete novice I must add,  I would have thought it was something to do with the battery but the dealership says it’s fine. 

Did you manage to get it fixed if so what was the problem as its happening to me now aswell

The OP hasn't been back to this forum since 14th June 2020, so I guess his car is still broke !

On mk2 sometimes the immobiliser problem is caused by faulty soldering in the instrument cluster or faulty/dirty/corroded canbus wire connectors on the way to ecu.  Perhaps with age the mk2 problems will now be appearing on the later cars. 

On my brothers 2009 focus the aa or rac (can’t remember which) said his immobiliser problem needed keys reprogramming . But it wasn’t that. It was the canbus wiring/connectors. 

Did you get this fixed I’m having the same issue??

6 hours ago, Chantellexo said:

Did you get this fixed I’m having the same issue??

Did you read the posts? 

On 11/30/2021 at 10:25 AM, unofix said:

The OP hasn't been back to this forum since 14th June 2020, so I guess his car is still broke !

 

My 2014 Cmax with keyless start has done this twice in about 9 years. I found that popping off the 5 cm cover on the side of the steering column and plugging the wireless key fob into the receptacle under the cover allowed the car to start. It also fixed the problem until it occurred again a few years later.