Mitsubishi Electric HP120TNU-A Service Handbook page 254

[8-3 Refrigerant Control Problems ]
2. Indoor unit LEV malfunction
Insufficient refrigerant flows due to LEV malfunction
(not enough opening).
3. Temperature reading error on the indoor unit piping
temperature sensor
If the temperature reading on the sensor is higher
than the actual temperature, it makes the subcool
seem smaller than it is, and the LEV opening de-
creases too much.
4 RPM error of the outdoor unit FAN
Motor failure or board failure, or airflow rate de-
crease, pressure drop due to clogging of the heat
exchanger leading to high discharge temperature
The fan is not properly controlled as the tempera-
ture cannot be precisely detected with the piping
sensor.
5. Insulation failure of the refrigerant piping
6. Long piping length
Excessively long piping on the high pressure side
causes pressure loss leading to increase in the high
pressure.
7. Piping size is not proper (thin)
8. Clogging by foreign object
9. The indoor unit inlet temperature is excessively
high.(exceeding 28°C [82°F])
10. Insufficient refrigerant amount
Protection works and compressor frequency does
not rise due to low discharge temperature
Refrigerant recovery operation is likely to start.
11. Compressor failure (same as in case of cooling)
12. BC controller LEV3 or LEV4 actuation failure
A drop in the low pressure that is caused either by a
blockage of liquid pipe or by a pressure loss and the
resultant slowing of refrigerant flow causes a tenden-
cy for the discharge temperature to rise.
13. LEV9 malfunction
Not enough refrigerant is provided to the indoor or
outdoor unit due to high-low pressure bypass that re-
sults from the malfunction of LEV9.
14. Open phase in the power-supply due to improper
power-supply wiring
HWE18220_GB
Cause Check method and remedy
Refer to the following page(s).[8-8 Troubleshooting
LEV Problems]
Check the thermistor.
Refer to the following page(s).
[8-7 Troubleshooting Outdoor Unit Fan Problems]
Confirm that the characteristic of capacity drop due
to piping length.
Change the pipe
Check the temperature difference between the up-
stream and the downstream of the pipe section that
is blocked. Since blockage in the extended section
is difficult to locate, operate the unit in the cooling
cycle, and follow the same procedures that are
used to locate the blockage of pipe during cooling
operation.
Remove the blockage in the pipe.
Check the inlet air temperature and for short cy-
cling. Change the environment where the indoor
unit is used.
Refer to item 1 (Compressor frequency does not
rise sufficiently.) on the previous page.
Refer to the following page(s). [6-3 Evaluating and
Adjusting Refrigerant Charge]
Check the discharge temperature.
Refer to the following page(s). [8-8 Troubleshooting
LEV Problems]
Refer to the following page(s). [8-8 Troubleshooting
LEV Problems]
Make sure that the power-supply wiring is properly
connected. (Refer to item (6) in section [6-1 Read
before Test Run].)
Possible open phase.
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