Mitsubishi Electric PUMY-P36NKMU3 Technical & Service Manual page 63

Check code
3121
Abnormal points and detection methods
When the thermistor temperature of -17°F[-27°C] or below has
continuously been detected for 3 minutes during heating operation
(during compressor operation), the unit makes an error stop and "3121"
appears on the LED1 and LED2.
The compressor restarts when the thermistor temperature is -13°F
[-25°C] or above.
If the unit is turned OFF, the outdoor temperature error will be canceled.
●Diagnosis of defects
Make sure to turn the power OFF before connecting/disconnecting any connectors, or replacing boards.
Check the outdoor air temperature.
Is the outdoor air temperature
-17°F [-27°C] or higher?
Check the resistance of TH7.
Is the resistance normal?
Check for pinched lead wire, condition of
wire coating and disconnected wire.
Is wire condition normal?
OCH733
Out-of-range outside air temperature
Diagnosis
Yes
Yes
Yes
Causes and checkpoints
Outdoor air temperature
Thermistor failure
Wire failure
Defective outdoor controller board
Use in operation range, which is -13°F
No
[-25°C] or more in heating mode.
Replace the TH7.
No
Remove the abnormal point.
No
Replace the outdoor controller board.
63
Remedy