Another update...
After around 10 run hours the mower started having issues starting. It would always crank over, but not always fire up. After around 20 hours of run time since the total no start condition, the mower was really acting up. It was getting very difficult to start and if it did start, it would often stall randomly with no warning.
Troubleshooting the electrical system again, I eventually discovered that when the engine would crank over but not start, voltage wasn't being sent to the EFI's cpu attached to the throttle body on the engine.
Further testing showed that the EFI relay, tapped to the wiring harness underneath the PBS module, was faulty. Sometimes it would work and switch power, sometimes not. When it did switch power, the voltage drop through the switching pins (pins 3 & 5 on the wiring diagram or 30 & 87) was up to 0.43VDC but would reduce as the relay would heat up when power was flowing through it. I switched out the relay with a new one and now the mower starts great and hasn't randomly stalled once!
The replacement relay I used was a genuine MTD unit, part # 925-1648A. The replacement part is not a like for like part. It's from a different manufacturer, it's a different colour and a little smaller. However the pin layout is exactly the same. The service advisor at Cub Cadet told me that they've just starting receiving these different relays and so far he wasn't aware of one failing. He also said that the shop has seen many of these original relay's fail. Seems to make sense on why a new relay is being sold.
I really hope that this is the end of this mower's electrical issues. Some of the issues were just wear & tear from normal use like dirty contacts, some from someone else's shoddy electrical troubleshooting and others from poor quality components selected by the manufacturer.
Besides these electrical issues, the mower has worked great and cuts grass well. It's just a shame that the electrical system is overly complicated with electronics for what a lawnmower requires and isn't more robust. Nobody likes a piece of unreliable equipment, especially for how much these things now cost. Its really a disservice to an otherwise good machine.
Hopefully my electrical issues can help someone else facing the same thing.
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