Take it to any auto parts store, most will do a free scan for the codes. If it's a major code it will need attention right away. You know the old saying, "a stitch in time, saves nine."
Service soon is not the same as "FIX IT NOW". You should be able to get a code scan and find out the problem. Check the obvious like oil level, gas cap, etc.
Search Ford Mustang Questions
Ford Mustang Experts