I am just wondering... a friend told me that a faulty gas cap can cause air in the tank.. is that true? also, can that cause the check engine light to come on?
There is always some air in the tank. The gas cap is part of the evaporative emission control system that is trying to prevent leaks of fuel vapor from escaping into the atmosphere.
Yes, it can cause the check engine light to come on. If you have a code reader you can plug into your ODBII port in the cab of your car there are some common codes that get thrown related to the evaporative emission control system.