Aprilaire Hvac Error Code E6

The E6 error code on an Aprilaire HVAC unit indicates a fault with the Low Temperature Sensor, also referred to as the Frost Sensor. This thermistor is responsible for monitoring coil or air temperatures to detect frost formation, which can signal unsafe or inefficient operating conditions. When the Internal Control board reads a signal from this sensor that falls outside the expected resistance range, it interprets this as either an open circuit (broken or disconnected sensor) or a short circuit (internal sensor failure or wiring fault), and triggers the E6 code to alert the technician.

The most likely causes of this fault are a loose connector at the Internal Control board or a sensor that has failed internally. Thermistors change resistance predictably with temperature, so measuring resistance across the sensor terminals is the definitive first step in diagnosing whether the sensor itself is at fault or whether the wiring and connector are the actual problem. An open circuit will typically show infinite resistance (OL on a multimeter), while a shorted sensor will show near-zero resistance, both of which fall outside any valid temperature-correlated reading.

To begin diagnosis, turn the ON/OFF toggle switch to the OFF position before opening the unit. Remove the side panel, insulation, and electrical box cover to access the wiring. Disconnect the Low Temperature Sensor connector and measure resistance directly across the sensor terminals. Compare the reading to the expected resistance range for the current ambient temperature. If the connector and wiring check out but resistance is still out of range, the thermistor itself has likely failed and will need to be replaced.