The Idle Air Control valve, also so as the IAC valve, controls the amount of air that enters an engine. It is located under the intake manifold. The IAC controls a vehicle's idle speed and can be easily worn out when a car's engine is idle for long periods of time. It also requires cleaning for optimum performance.
The most common symptom of a bad IAC valve is stalling. This typically occurs when the driver releases his foot from the gas. After the car stalls out, it usually starts up again with no problem. In cases of intermittent stalling such as this, the IAC is usually defective. In this instance the IAC needs to be either replaced -- which is expensive -- or fixed -- which is time consuming, but relatively cheap.
A cold start is when a car starts with no problem, then quickly dies. This is typically a sign of a dirty IAC valve. A lot of dirt and grease can build up in an IAC valve, which can diminish the performance. Often an IAC valve needs to be cleaned three or four times with a degreaser. If you are still having problems, either the IAC needs to be replaced, or there is another problem causing your vehicle to cold start.
Sometimes a vehicle may appear to have a bad IAC valve when really it is just the effect of cold weather. If a vehicle refuses to start and it is particularly cold outside, a quick fix might be all it needs. Try warming up the IAC with a hairdryer. Assuming the weather is the problem, once the IAC has warmed up, the engine should easily start.