2000 Ford Mustang Repair Question
Ford Mustang Battery Problem
You have an intermittent "no-crank" condition. There's no point in thinking about the fuel pump or any other unrelated stuff. As for "never having that problem before", that is true for every problem on every car in the history of cars. All problems have a point when they show up for the first time.
The first thing you need to do is take note of any other symptoms or observations when the problem occurs again. In particular, exactly what sounds do you hear when you turn the ignition switch to "crank"? Do you hear a single rather loud clunk under the hood, and a second one when you release the key? Turn on the head lights or dome light first, then try to start the engine. Do the lights start out dim? Do they start out bright then go out or dim when you try to start the engine? Do they stay bright when you try to start the engine?
Some things you can check yourself are the battery cable connections and the other ends of those cables. The red positive cable bolts to the starter relay near the battery. Check the larger copper nuts on those studs to be sure they're tight. They often work loose and cause intermittent starting problems. Be careful when using any type of wrench on those connections that it doesn't contact anything else metal on the car at the same time. Doing so will cause horrendous sparks.
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