Fixed. I found a broken wire at the bottom where the zip ties had been pulled too tight and had worn the wire in half over many years.

Troubleshooting procedure I used:
1) Removed top of machine (three bolts on back) to access the controller.
2) Pulled off the two connectors that had two black wires in them. The temp sensor harness is two black wires.
3) Measured resistance at each connector between the two pins. First one was low ohms, <10. Second one was open. Expected for the temp senor would have been in the 10k-30k ohm range - not low ohms or open.
4) Used ohm meter in buzz mode to determine that one of the wires in the second connector was zero ohms (good) and one was open (bad).
5) Visually inspected connectors, they looked good.
6) Clipped zip ties to extract connectors for a better look and found broken wire at second zip tie in the bottom of the machine.

Spliced, soldered and heat shrink to fix. Verified from the top with ohm meter and got about 17k ohms.