Cause of error code E40

Return water temperature sensor sees temperature higher than 200°F.

Solution for error code E40

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

2. Unplug molex connector on return sensor. Is there moisture on liquid on metal pins? Liquid might be water or antiofreeze solutions.
Yes: Remove all liquid from connector. Check for leak. Repair all leaks. 
No: -> 3

3. Is heat exchanger pump properly oriented? ( pump arrow pointing down) 
Yes: -> 4  
No: Install heat exchanger pump in proper orientation.

4. 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.