When you try and start the engine and it does not go, is there any noise. Your listening for a moderately loud click. If you dont hear anything then the computer could be blocking start or you have a bad connection to the starter relay. They use to call it a solenoid and may still but I stopped working on engines when they got too computerized. This relay takes a small current from the key switch and activates a monster relay(the click) that can handle the hundreds of amps needed to spin the engine. If you hear the click and nothing else happens then check the connections from the solenoid to the starter and the grounds to the chassis. If all these things are good you may have a bad spot on the starter. Then its just hit or miss if the starter stops on the bad spot or not. To find the solenoid follow the + battery cable (big one) Solenoids are usually mounted on the fender wall.
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