Indoor Unit Temperature Thermistor Failure; Circulation Water Freeze Protection - Mitsubishi Electric EHST17D-VM2D Service Manual

Error code Title and display conditions
P1/P2/L5/LD

Indoor unit temperature thermistor failure

Note: The thermistors subject to failure can be
checked in "Request code: 567" in
"Running information.
<DHW/Heating/Cooling/LP/FS/OS>
Error code displayed when thermistor is at open or
short (see table).
Exceptions
Error code will not be displayed for TH2; During
defrost and for 10 minutes after defrost operation.
Error code
P1 TH1A/TH1B
P2 TH2
THW1
THW2
THW5B
L5 THW6
THW7
THW8
THW9
LD THWB1
L6

Circulation water freeze protection

<DHW/Heating/Cooling/LP/FS/OS>
Error code displayed when THW1 detects a
temp. ≤ 1ºC for 10 consecutive seconds or
THW2 detects a temp. ≤ 3ºC for 10 consecutive
seconds.
Exception
Error code will not be displayed if;
FS function is disabled,
For 10 minutes after water circulation pump1 is
switched on.
OCH714A
1.
2.
3.
4.
5.
Thermistor
Symbol Name
Room temperature thermistor
Liquid temperature thermistor
Flow water temperature thermistor
Return water temperature thermistor
Tank water temperature thermistor
Zone1 flow water temperature thermistor
Zone1 return water temperature thermistor
Zone2 flow water temperature thermistor
Zone2 return water temperature thermistor
Boiler flow water temperature thermistor
1. Insufficient system head
2. Reduced flow in primary water circuit
3. Valve operation fault
4. 2-way valve (local supply) actuator
5. 3-way valve actuator fault
6. THW1 has become detached from its
7. THW1 or THW2 fault
8. FTC board failure
Possible Cause
Connector/terminal wire has become
detached or loose wiring.
Thermistor fault
FTC board failure
The thermistor on the wireless remote
controller or the main remote control-
ler may be defective. (when Room
temp. is chosen for the Heating opera-
tion and when Main remote controller
or Room RC 1-8 is chosen for the
Room Sensor setting in the Initial
setting)
Incorrect setting of the DIP switch(es)
Open detection
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−39ºC or below
−40ºC or below
Due to 1 or more of the following;
Faulty pump, insufficient air purge,
blocked strainer, leak in water circuit
fault
holder.
50
Diagnosis and action
1. Visually check the terminals and connec-
tions and reattaches appropriate.
2. Check resistance of thermistor against
table in "10-6. Checking Component Parts'
Function".
Compare FTC detected temperature to
hand held detector.
3. Replace board.
4. Replace wireless remote controller or main
remote controller.
5. Check the DIP switch setting(s).
Short detection
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
88.5ºC or above
140ºC or above
1. Refer to table in "10-6. Checking Compo-
nent Parts' Function" to determine if system
pump meets requirements.
If more head required either add a pump of
the same size or replace existing pump with
capacity model.
See "11. DISASSEMBLY PROCEDURE" for
how to replace pump.
2. Check circulation pump (See "10-6.
Checking Component Parts' Function" for
how to check).
Open purge valve to remove trapped air.
Check the strainer for blockages.
Check the primary water circuit for leaks.
Check that the flow amount is within the
recommended range.
3. Check valves on primary water circuit are
installed level.
4. Electrically test to determine fault.
5. 1) Electrically test to determine fault.
2) Operate 3-way valve manually using the
main remote controller. (Refer to
<Manual operation> in "9-4. Service
menu".)
3) Replace 3-way valve coil.
4) Replace 3-way valve. (Refer to Procedure
6 in "11. DISASSEMBLY PROCEDURE".)
6. Visually inspect location and reattach as
necessary.
7. Check resistance of thermistor against table
in "10-6. Checking Component Parts'
Function".
Compare FTC detected temperature to
hand held detector.
8. Replace board.