If your Mustang is 1995 or earlier, I found a Ford code checker, Innova Electronics Model #3145 digital at Kragen for 35 bucks. I thought I had a fuel supply problem when my 92 LX 2.3L died on the highway. Tediously ruled that out, bought the code checker and 5 minutes later had my answer, a code 211, bad crankshaft position sensor (PIP) I highly recommend this inexpensive tool for any Ford owner. I don't know if the code checker for 1996 up is more expensive, but it might be worth it anyway.
Good luck!