Mitsubishi Electric MXZ-SM72TAM-U1 Service Manual page 49

3121: Out-of-range outside air temperature
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 tempera-
ture is -14°F (-25.6°C) or above.
• If the unit is turned OFF, the outdoor temperature error
will be canceled.
Diagnosis of failure
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?
OCH800A_49
No
Yes
No
Yes
No
Yes
Causes and checkpoints
• Outdoor air temperature
• Thermistor failure
• Wire failure
• Defective outdoor controller board
Use in operation range, which is -14°F (-25.6°C)
or more in heating mode.
Replace the TH7.
Remove the abnormal point.
Replace the outdoor controller board.