idle speed wrong

I have a strange problem in my r19 16v phase 1
-when the car is moving and no gear is selected the motor idle speed is 1500 rpm
-after I stop the car the motor idle speed goes to 950 - 1000 rpm
I suspect that the air temperature sensor to be broken (the cold start injector to be started because of it) and this could cause the wrong idle speed - I don't know how to test it

Has anyone encountered this problem?
10x