Mitsubishi Electric PUMY-P36NKMU3 Technical & Service Manual page 62

Check code
1508
(EF)
Abnormal points and detection methods
If 4-way valve does not operate during heating operation.
When any of the following temperature conditions is satisfied for 3 minutes
or more during heating operation when the outdoor temperature is −4°F
[−20°C] or more:
1. TH22j − TH21j [ −18°F [−10°C]
2. TH23j − TH21j [ −18°F [−10°C]
3. TH22j [ 37.4°F [3:]
4. TH23j [ 37.4°F [3:]
Note:
For indoor unit, the abnormality is detected if an operating unit satisfies the
condition.
●Diagnosis of defects
Make sure to turn the power OFF before connecting/disconnecting any connectors, or replacing boards.
Is the connector for outdoor
controller board or 4-way valve coil
disconnected or loose?
Disconnect the connector for outdoor
controller board or 4-way valve coil to
check the resistance.
Is the resistance detected?
Check the voltage and appearance of the
outdoor controller board.
Is the detected voltage normal?
Is it free from any trace of over-
heating or burning?
OCH733
4-way valve trouble in heating mode
Diagnosis
No
Please refer to "8-7. HOW TO CHECK
THE PARTS" for ohm values.
Yes
Yes
Causes and checkpoints
4-way valve failure
Disconnection or failure of 4-way valve coil
Clogged drain pipe
Disconnection or loose connection of connectors
Malfunction of input circuit on outdoor controller board
Defective outdoor power board
TH21: Indoor intake temperature thermistor
TH22: Indoor liquid pipe temperature thermistor
Reconnect the connector or connect it
Yes
tightly.
No
Replace the outdoor multi controller circuit
No
board.
Replace the outdoor power circuit board.
62
Remedy
Replace the 4-way coil.