My niece has a 2002 Mercury Sable that stalls at stop lights when the A/C is on but is ok if she turns the A/C off. Is there a sensor or switch that compensates for the A/C being on to increase the RPM an prevent stalling at stops.
The most suspected part to cause a condition like this would be the IAC valve (Idle Air Control). A scan tool could verify this by the mechanic seeing a high duty cycle on the IAC parameter.
Try to unplug and plug in the connector at the IAC valve a few times, sometimes a poor connection could be responsible. If this doesn't work then go to the next step.
What you would need to do is take the air intake hose off at the throttle body and clean all the carbon around the throttle plate with an authorized factory cleaner and a rag. With the engine off and the throttle plate wide open make sure all the carbon is removed inside the throttle body. If this does not fix the problem then you may have to replace the IAC valve. Cleaning the IAC valve will not make a difference.
The problem could also be caused by a vacuum leak like a cracked vacuum hose like the PCV valve hose or brake booster hose. Also check the smaller hoses for cracks or being disconnected at or around the intake manifold.