Malfunction light

If that is the check engine light, there is well over 2,000 potential defects the engine computer can detect. Only a few of them refer to "small leaks" or "gross leaks", in the fuel vapor recovery system, and the gas cap is just one of many potential causes for setting those codes. The place to start is by having the diagnostic fault codes read and recorded. Those will indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. The people at many auto parts stores will read engine codes for you for free.