I doubt a sensor problem, as he has just replaced it ..he was thinking the same ..
The 300e has to get to 105-107 to trip N22, so until you know if that trip point is reached, true diagnosis can't be completed..
If a trip is found at spec without the mod harness, the next test is with the harness mod plugged back in. If the fan still has to go to spec [105-107] than the mod [ be it open, bad connection, R value, etc] is at fault . [ as if it was not there.. i.e.-open circuit]

Verify aux fan operation w/o mod .
If no change with mod, Check mod circuit..