N14 celect plus erratic idle

[schISM]

Had to go reread what you meant by, “No reading in the cab for ambient air temp.” I thought you were saying there “is” a display in the cab but it wasn’t reading anything. No matter though, at this point you can clearly see the circuit for ambient air temp is isolated solely to one sensor. Now find the sensor and unplug it. If your reading in Insite stays at 40 degrees you know the circuit is faulty. Now make sure the circuit has a 5 volt feed, if so you could replace the sensor to see if that’s the issue or ohm out the signal wire to the proper pin on the ECM engine side.