Safety first: Isolate power before accessing the control compartment. Do not operate the furnace with sensors bypassed.

Steps:

Locate the NTC sensor: Refer to the wiring diagram to find the Danfoss NTC temperature sensor mounted on the heat exchanger or supply air plenum.
Inspect wiring: Check the sensor cable for cuts, abrasions, or signs of overheating. Ensure the plug is fully seated on the control board.
Check for corrosion: Look for moisture or rust around the sensor mounting point. Dry the area and clean light corrosion with a soft brush.
Basic resistance check (if you have a meter): With power off and sensor disconnected, measure resistance across the sensor leads. Compare to the Danfoss temperature/resistance chart. A reading of 0 Ω (short) or infinite (open) indicates a failed sensor.
Replace if faulty: If the sensor is defective, replace it with the exact Danfoss NTC sensor model specified for your furnace. Secure the new sensor firmly against the heat exchanger surface.
Reset and test: Reconnect wiring, restore power, clear the E5 error per the manual, and run a heating cycle to confirm normal operation.

Furnace displays E5 and will not start, internal temperature sensor fault indicated

Possible Causes