"Engine Transmission light/Power train Light" up and it goes into "Limp Mod?

HI,

I have a Ford Fiesta Climate 08 TDi 1.4 Turbo, it has only done 37,000 miles.

Recently i have been getting the "Engine Transmission light/Power train Light" up and it goes into "Limp Mode". This has been happening over the last month and a half. We have plugged the diagnostics into it but it only comes up with the glow plugs error.

Just after the error started coming up my "back box" was replaced as it came off, this stopped the error for a day.

The error comes up at lots of different times for example: When i first turn on the car, when i'm reversing, it has come up when i have been in ever gear (so not due to speed) and its not how i drive it as this has come up when my partner drove it.

Hope you can help.

Claire Harris,