Safety: Shut off power to the HVAC system at the breaker before working on low‑voltage wiring.

Inspect outdoor sensor: Locate the Honeywell outdoor sensor (often mounted on an exterior wall). Check for cracks, water entry, or corrosion. If the housing is full of moisture, the sensor is likely shorted.
Check wiring connections: At both the thermostat and sensor ends, ensure the two sensor wires are not touching each other or any metal surfaces. Tighten terminal screws.
Test for short: With both ends disconnected, measure resistance between the two sensor wires and between each wire and ground. Near‑zero ohms between wires indicates a short in the cable or sensor.
Isolate the fault: Disconnect the sensor from the cable and test the sensor alone. If the sensor reads near‑zero ohms, replace the sensor. If the sensor is normal but the cable is shorted, replace the cable run.
Clear error: After repair, restore power and verify that the E4 code clears and outdoor temperature displays correctly.

Thermostat displays E4 and may show incorrect outdoor temperature

Possible Causes