Mitsubishi Electric PUMY-P36NKMU4 Technical & Service Manual page 71

Check code
3121
Abnormal points and detection methods
When the thermistor temperature of -24°F[-31.1°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 -14°F
[-25.6°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
-24°F [-31.1°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?
OCH811A
Out-of-range outside air temperature
Diagnosis
Yes
Yes
Yes
Causes and checkpoints
Outdoor air temperature
Thermistor failure
Wire failure
Defective outdoor controller board
TH7: Thermistor <Ambient>
Use in operation range, which is -14°F
No
[-25.6°C] or more in heating mode.
No Replace the TH7.
Remove the abnormal point.
No
Replace the outdoor controller board.
71
Remedy