What could be causing my 04 Beetle turbo to get bad gas mileage and hrd starting on 2004 Volkswagen Beetle

Diagnostic showed a bad coolant sensor... car will start up fine cold, but after driving and turning car off a couple times, it will not start. After letting it sit for 15-20 minutes, it will start again.

Asked by for the 2004 Volkswagen Beetle
could be several issue incl coolant sensor circuit or mass air flow senor,seek diag and est