How to Fix / Troubleshooting

Safety first: Disconnect power before removing covers.

Steps:

1. Access the evaporator coil: Remove the front panel, filters, and sometimes additional covers to expose the aluminum coil.
2. Locate the coil thermistor: It is usually clipped to the coil fins or inserted into a small pocket on the coil.
3. Check attachment: Ensure the sensor is firmly attached to the coil and not hanging loose. Re-clip it if necessary.
4. Inspect wiring: Follow the sensor wires back to the PCB, checking for cuts or loose connectors. Reseat the connector.
5. Replace if faulty: If the sensor is physically damaged or tests open/short with a multimeter, replace it with a Panasonic indoor coil thermistor.

If H21 remains after sensor replacement, the indoor PCB may need replacement by a technician.

The unit stops and displays H21; this is often associated with abnormal indoor coil temperature readings.

Possible Causes