Thermostat shows E2 and room temperature reading is clearly wrong (e.g., shows -20°C or 99°C) or fluctuates rapidly.

Safety first: Isolate power to the thermostat at the breaker or remove it from the powered backplate. Do not work on live low-voltage circuits.

Steps to diagnose and fix:

• 1. Visual inspection: Remove the thermostat cover. Inspect the sensor area for moisture, corrosion, or debris. If condensation is present, gently dry the area with a lint-free cloth and allow the unit to air dry fully.
• 2. Check for contamination: Look for dust, metal filings, or solder splashes bridging the sensor pins. Use a soft brush and isopropyl alcohol to clean the PCB around the sensor.
• 3. Measure resistance: With the sensor disconnected (if possible), measure resistance with a multimeter. A reading near 0 Ω indicates a shorted sensor. Compare to Danfoss specifications for your model.
• 4. Inspect wiring harness: If the sensor is remote or wired, check the cable for crushed insulation or pinched sections where conductors may be shorted together. Repair or replace the cable if damaged.
• 5. Replace sensor or thermostat: If the NTC is integrated and visibly damaged (burn marks, cracked body), replace the thermostat head unit. If it is a plug-in sensor, install a new Danfoss-compatible NTC sensor module.

When to call a professional: If the short appears to be on the PCB or you cannot identify the cause, a replacement thermostat or professional diagnosis is recommended.