Return water temperature sensor is shorted. '0' Ohm
1. Disconnect harness from return temperature sensor. Measure resistance across sensor terminals. Is resistance less than 50 ohms?
Yes: Replace return water sensor.
No: -> 2
2. Disconnect harness from controle module J5. Measure resistance between brown wires at terminals J5-4 and J5-12. Is resistance less than 50 ohms?
Yes: Replace wires
No: Replace control module.