Nice write up by bdub, I just bumped thst thread to add drivel about the shift cables

Google FPS.
Use only Bosch or Genuine Volvo.
It is a 5 min job using at Torx key.

A bad FPS will never throw an error code.
You can always get a junk yard FPS to try.
But it is so inexpensive (Bosch 0261230110 is ~ US $42), it is good to try.

PS: if you have a Fuel Gauge device, then it will be clear.