Found on a forum:
Ford Focus cranking but not starting

Hello. I have a problem with a Focus TDCi, and am wondering if anyone reading this has encountered 
something similar. It involves the glow plug warning light.
When I went to start the car the other morning, I turned on the ignition and waited for all the usual 
lights to go out. I then turned the engine over. After about 5 seconds it hadnt started and the glow 
plug warning light was flashing.
I then released the key, which left the ignition still on, and the warning light still flashing.

I then turned the ignition completely off and tried again. This time the light flashed as soon as the 
ignition was turned on. I repeated this procedure a few more times and eventually everything went 
back to normal and the car started as usual. I had no more problems for the next day or so.

Then tonight I went to start up. Waited for all the usual lights to go out again, and turned the key to 
crank the engine. It turned over for about 10 seconds before I gave up. The glow plug light was 
flashing again. I turned the ignition off and on again and it was back to being completely normal.
Has anyone seen this before, or have any suggestions as to what it might be?
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On fords, the glow plug light flashes when the engine management detects a fault.
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A flashing glow plug light on the diesel indicates a problem with the immobilizer, specifically 
transponder signal not detected. i.e. The ECU didnt pick up the correct code from the key to 
re-mobilize the vehicle.

I would try the spare key for a few days. If the fault doesnt re-occur then you have a possible fault 
with the main key and may need a replacement. (are there lots of other keys on the same ring- 
this will upset the signal)
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Well... the car was in for a diagnostic this morning. This did not indicate any problems, so in light 
of recent experience with a car with the same problem, they reprogrammed the module and keys
and reckon this should cure the problem.

It hasn't actually happened again since my original posting, so it could be a while (or never!) before 
I find out if this actually DID fix it. I now carry my spare key about with me just in case, so I think 
I'll keep this up for a while longer anyway.

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