Coil light and limp mode

[TJ1]

My A4 2L tdi coil lights keeps coming on then goes into limp mode. I have had diagnostics a few times which indicated a problem with fuel sensor. It has been to the garage to see if they can find the problem,  I have had the sensor replaced, the relay for the glow plug replaced but. I'm at a loss what to do next, does anyone have any ideas? 

[Steve Q]

Could it be an issue with an injector, cam sensor, dpf or egr.