My car was fixed by them as the warning was coming...

My car was fixed by them as the warning was coming up "Engine Hot, Drive Moderately". However I think it was just a sensor problem as this was up even if you start the car after a week in freezing cold. They diagnosed that the expansion tank had a crack so I was charged around 1500$ for the replacement. 6 months forward, the warning came up again. I brought the car back with the impression that it must be sensor and no I am suspecting whether they really replaced that expansion tank? To my dismay, without even looking at it the agent told me it could be something else related. And now they sent me a report :
Perform brake system fluid exchange (Found brake fluid excessively dirty/contaminated) x Fail $250.00 Diagnose oil leak (Found engine oil leak) x Fail $179.00 FAULTS STORED FOR TURBOCHARGER COOLANT PUMP CONTROL. REC REPLACING TURBOCHARGER
COOLANT PUMP (C/S VEHICLE OVERHEATING) Fail $798.65
REC REPLACING TURBOCHARGER COOLANT RESERVOIR (ON INSPECTION FOUND COOLANT SCREEN CLOGGED)
All this totaling over 3000$
My simple question to them is, lets assume what they diagnosed is correct, why does the warning come up even when the car is cold. They had no answer to that. So I think its just a sensor gone bad. Its very easy for them to name so many technical terms to unsuspecting people but I think the report is made up. So be aware if you go to them I am not going back.

Another point: To re-diagnose something they fixed they are charging $180. Is it really fair? I wish there are some serious laws around these type of mechanics