Usually, if something's wrong with the car's system, it should show an error message on the dashboard or the mechanic can see it. Have you tried cleaning the sensors?
It might be that the car's system needs to be reset, like a recalibration. If your car is still under warranty, you shouldn't have to pay for this.
I think you should try taking your car to your dealer. If that doesn't work, you can call the manufacturer directly and tell them about this dealer. Explain what's happening and give them the dealer's name. The repairs should be covered by the warranty.