TDi 110, year 2001; just had 140,000 service 2 weeks ago. Recently have been doing only short town trips but yesterday on first long run the yellow pre-heater light started flashing on the dash. Took the car to a friend with OBDII reader but no error codes were showing. Does anyone have any ideas, could it be due to build up of oil deposits somewhere caused by short trips in cold weather ?

You need to get it on Vagcom/VCDS diagnostic, as there are a lot of VAG codes that OBDII code readers don't recognise.

Very much doubt it's anything to do with short trips.

You need to get it on Vagcom/VCDS diagnostic, as there are a lot of VAG codes that OBDII code readers don't recognise.

Very much doubt it's anything to do with short trips.

+1

Also, do your brake lights work?

Brake light switch is the most common fault for the glow plug light, failing that you need a VCDS or dealer scan

Thank you for the hint about the brake light switch, it went intermittent 18 months ago and I thought I fixed it. Guess it will need a new one now.

It doesn't help that the glow plug light is the "something has gone wrong" light and there are potentially a lot of things that can make it come on.