Coolant temp gauge sticks around 80 unless im at motorway cruising speeds or.. stopped in traffic, pointing slightly downhill for a few mins. In either of those circumstances it will climb to the 90 mark dead centre. Im not sure if im down on mpg as ive been more mway miles recently so hard to tell. Coming due a service shortly so woñdering which is the likelier culprit.. a thermostat sticking or failing cts?

1.9 diesel ALH btw cheers guys!

Sounds like stat sticking so it's over-cooling hence dropping down to 80. At least that's what mine was doing, I changed the CTS while I was there though to be sure. Hang on, mine would drop temp when off gas, going downhill etc, and go back to 90 when back on gas, is that the other way round to yours? I'm confused lol. If it was failing CTS there may be other running issues, power loss, misfire etc. If overcooling you should have been running rich and burning a bit more.

I had exactly the same as YellowCar - temp was below 90 when cruising but would go up to 90 in traffic. Replacing the thermostat fixed it. I also replaced the CTS at about the same time but it didn't make any difference.

I had the same did cts as it's much easier to get at but made no difference though then did the stat 2 weeks later which sorted it and now its about 2/3 mpg better on fuel aswell which is a nice little bonus

Mmm... great. Might just get both sorted anyway and be done with it while coolant is dropped anyway! No point doing one then the other a month latee!

Think it is stat but doing cts is easy and then you'll know where you're at. Use gen, some have used gates with some success but I'd rather stick to genuine vag ones.

For sensors and the like genuine is the only way, so itl be down the dealers during the week methinks and add them to the service job list...