ClimateMaster water-to-water unit TMW 036. Closed loop. I cut JW3-FP1 wire. It worked beautifully for 18 months, then it tripped out - LED and alarm: Cycling Code 4. FP1 fault. Check water flows, both ok. I ordered a new FP1 thermistor, and installed it. Still failing, Cycling Code 4. Then I took the FP1 sensor off the refrigerant line. I left the gray wires undisturbed, still plugged in to their socket. It worked beautifully! While it was running, I checked the temperature where the sensor for FP1 is supposed to be. The temperature measured + 20 F. What is the matter, control board failure? Source water in: 43 F; Source water out: 40 F; Heating water in: 90 F; Heating water out: 93 F; and the refrigerant pressures are where they are supposed to be!

Put the thermistor back and call a pro

Ditto

Sounds like a refrigerant charge or metering error. The reported water temperatures are inconsistent with 20*F anywhere in system absent a charge or metering problem.

"refrigerant pressures are where they are supposed to be!" should be scrutinized and subject to analysis of the state of refrigerant at various points throughout the system - superheated, subcooled (and if so by how much) or a saturated mixture.