E2 on indoor display, unit runs briefly then shuts down, poor temperature control

Safety first: Disconnect power at the breaker and indoor service switch before opening the air handler or fan coil.

• Step 1 – Access the evaporator coil: Remove the blower/coil access panel on the Day & Night air handler or cased coil. Locate the small sensor clipped or strapped to the evaporator coil tubing.
• Step 2 – Check sensor placement: Ensure the thermistor is firmly attached to the coil surface and not hanging loose. Re-clip or re-tape it securely if needed.
• Step 3 – Inspect wiring: Follow the thermistor wires back to the indoor control board. Confirm the plug is fully seated and wires are not pinched or cut. Reseat the connector.
• Step 4 – Resistance test (if equipped): With power OFF, disconnect the thermistor and measure resistance with a multimeter. Compare to Day & Night service data (typically 10k–20k ohms at room temperature). If reading is open, shorted, or far out of spec, replace the sensor.
• Step 5 – Reset power: Restore power and run the system. If E2 returns and the sensor tests good, the indoor PCB may have a failed sensor circuit.