Customer had a 2010 DD15 engine. The Fan would not kick on at 220 degrees, even though it was programmed that way. The customer put a new ECM on the truck (MCM technically), and it didn't fix the problem. They checked solenoid resistance, it was within spec.
The solution was to change to both coolant sensors. The upper controls functions such as a fan on, fuel mappings, etc... This issue can be present on a regen, cause low power issues, and also happens with DD13 engines. You usually find the sensor pitted into the sensor probe. It reads fine at operating temperatures, but stops moving up once it got over 215 F.
