Cause of error code E40

Return Temp. 

Solution for error code E40

1. Is correct harness connected to return sensor (2 Brown Wires)?
Yes: -> 2
No: Correct wiring.

2. Is Heat Exchanger Pump oriented properly? (Pump Arrow pointing down?)
Yes: -> 3
No: Install heat exchanger pump in proper orientation.

3. Disconnect harness from Return Water Sensor. Measure resistance using digital ohm meter. Is resistance between 950 to 33,000 ohms? 
Yes: Replace Control Module. 
No: Replace Return Water Sensor.