Start by preforming these test:
Engine start/stop switch verification;
Disconnect the Yellow/Red wire of the start/stop switch. Using an ohmmeter, connect test probes to the Yellow/Red wire and to ground. Measure resistance, it must be an open circuit (switch is normally open). Depress and hold switch, the ohmmeter should read close to “0” ohm.
Safety Lanyard Switch Verification;
If 2 short beeps are not heard when installing the safety lanyard, disconnect the switch wires.
Safety lanyard removed;
connect the test probes to switch Black and Black/Yellow wires. It’s an open circuit, there should be no continuity.
Connect one test probe to the White/Gray wire and the other test probe to the switch terminal. Measure resistance, it should be “0” ohm. Connect one test probe to the Black wire and the other test probe to the switch ring. Measure resistance, it must be close to “0” ohm.
Safety lanyard on switch; connect the probes to switch Black and Black /Yellow wires. Measure resistance, it must be “0” ohm.
If all that is correct then you will want to check 12v out to starter solenoid and that it is not broken or grounded and or a poor connection such as corrosion. Not only do you have the fuses in the mpem but also in the black electronics box. From there if you have no 12v out from the mpem to the solenoid but the dess post/system is good as well as start/stop switch and all wiring then mpem is generally suspect, I do see you swapped mpem, yours using the dess system means the key must be programmed to the mpem, this means the machine you "borrowed" the mpem from you must use that dess key post cap to test with as it is programmed to it and vise versa, if you tried the other mpem but used the key cap that goes to the non working pwc then that would test incorrect and make you assume it was not the problem.
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