my mates old 1.6 polo cl did about 28mpg he fitted a new coolant temp sender and did over 40 mpg after that so i would look to see if yours is ok as vw's seem to go through them alot
would a engine code come up if this was knackered? tell tale signs?
It will do, but it's not only sensors that affect the economy of the engine, wear and tear will often deteriorate it too. My engine was on 150k miles and I was getting around 27-32mpg average around town driving very carefully but my compression was down a bit - albeit consistently down across all 4 cylinders with a lot of blow-by which can only have hurt my mpg before.

This was how my piston was looking before I cleaned it up!
Also found that 2 of my exhaust valves were not seated as nicely as it is now, so again losing compression there. All adds up, but with a fresh motor I am anticipating healthy mpg
