My car is throwing a P0172 error code - Fuel System Too Rich, nothing else.

I have taken it to a independent garage and dealership, but none seem to be able to fix and speculatively say we can do this for £x. No one can offer a diagnosis and so it falls to me with not much money to find a resolution.

The only issue i can see from my own obdii scanner is that long term fuel trim is -25% on idle everything else seems fine. O2 sensor is switching between rich and lean keeping short term at 0. When i rev the car to 3000 the long term trim reduces and short term increases.

History: Cambelt replaced in September along with alternator belt and rocker seal. TPS replaced in October. Had my exhaust system inspected by Kwik Fit to cheak for air leak near 02 sensor at front and its air tight.

Drives ok have done both short and long journeys no problem, the number of miles car can do has dropped slightly, car used to make a hunting sound but the dealerships pcm update helped resolve this.

Any ideas?