Start up

Suspect the crankshaft position sensor first and the camshaft position sensor second. If the Check Engine light has turned on while driving there will be a diagnostic fault code stored in the Engine Computer that will say which circuit has the problem. When it doesn't start, check first for spark. If it is missing, (and you don't smell raw fuel at the tail pipe), measure the voltage on the dark green / orange wire to the ignition coil, any injector, or either small wire on the back of the alternator. You should see battery voltage for just one second after a helper turns on the ignition switch. What's important is that voltage must come back during engine cranking. If it does not, the Engine Computer is not turning on the automatic shutdown relay, typically in response to the missing signal from one of those failed sensors. It is very common for them to become heat-sensitive when they fail, and they will work again when they cool down.