OK, I have seen more then a few fail. BUT the biggest failure was always, operator/mech "Knobdicking". For those of you who are new to Knobdicking, it means:
1. "Adjusting" them for no reason.
2. Not reading the TM for the proper testing, adjusting and reinstallation of the MPU.
3. Not properly tightening the lock nut.
These three reasons are 95% of the reason there are MPU faults. All result in no signal, or a ground off MPU end.
I did not list "problems". Problems are:
1. Dirt, trash and other "impure" things on the end of the MPU.
2. Broken wires.
3. Failure to set ones Meter to AC volts when "testing".
But as long as you test, remove, clean, reinstall, adjust and retest right. The MPU will last a long time. I still would have one in my spares.