Return sens shorted.
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 Control 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.