Engine will start and stop immediately and continue to restart. The only way to go is to hold the gas pedal down a little and drop it in gear. Where is the sensor that controls the idle?

What year and engine?

engine will start and stop immediately and continue to restart. The only way to go is to hold the gas pedal down a little and drop it in gear. where is the sensor that controls the idle?

If you're talking about the Idle Air Control Valve (IACV) you'll find it mounted to the throttle body. I'd try cleaning it first. See your local parts supplier and read the instructions on the label.

Clean the throttle plate, throttle body, and Idle Air Control Valve (IACV) with Throttle Body Cleaner. If the problem persists, replace the IACV, which is likely located on top of the throttle body. Afterwards, clean the air box and replace the air filter.

Thanks to all. A couple of taps and it idles again.

That's an indication that the stepper motor, located inside the IACV, is going. It regulates air flow during idle. Try cleaning the IACV--if the problem returns, replace it.