N14 celect plus erratic idle

[schISM]

The 40 degrees fahrenheit sounds like a default value the ecm will use if it’s not receiving a signal from the sensor. The fact that it straightens out when you activate the cylinder cut out test would lead me to believe the issue wouldn’t be hard parts but maybe the sensor for ambient or the wire loom to the sensor was damaged during the recent work you performed. I’d trouble shoot the ambient temp. circuit till you get a correct reading and see if that helps....either way it couldn’t hurt.