*****INTERNAL*****30 R_D proof- 26.10.2022 / 09:15:46- VaillantGroup\DOC-agt\Brand\VIH_QW_190_6\ENG\OI-II_GB_0020291525
0020291525_04 Installation and maintenance instructions 59
Code Meaning Cause
Remedy
F.741 Building circuit: Return temp. too
low
– During the de-icing, the return tem-
perature falls below 13 °C
– Ensure the minimum installation
volume is maintained; if required,
install a series return cylinder
– The fault message is displayed until
the return temperature increases to
above 20 °C.
– Activate the electric back-up heater
in the product's control panel and
in the system control in order to
increase the return temperature. The
compressor is blocked while the fault
message is present.
F.752 Fault: Frequency converter –
Internal electronics fault on the in-
verter PCB
– Mains voltage outside of 70 V–282 V
–
Check the integrity of the power sup-
ply cables and compressor connec-
tion cables
The plugs must audibly click into
place.
– Check the cable
– Check the mains voltage
The mains voltage must be between
195 V and 253 V.
– Check the phases
– Replace the frequency converter, if
required
F.753 Connection fault: Freq. conv.
n.recogn.
– Missing communication between the
frequency converter and the outdoor
unit's control PCB
– Check the integrity of the wiring
harness and plug connection and,
if required, replace them
– Check the frequency converter by
actuating the compressor safety
relay
– Read the assigned parameters for
the frequency converter and check
whether values are displayed
F.755 Fault: 4-port valve position not
correct
– Incorrect position of the 4-port di-
verter valve. If, in heating mode, the
flow temperature is lower than the
return temperature in the building cir-
cuit.
– The temperature sensor in the EEV
environment circuit displays an incor-
rect temperature.
– Check the 4-port diverter valve (is an
audible switching available? Use the
sensor/actuator test)
– Check that the coil is positioned
correctly on the 4-port valve
– Check the wiring harness and plug
connections
– Check the temperature sensor in the
EEV environment circuit
F.774 Sensor fault: Air inlet temp. – The sensor is not connected or the
sensor input has short-circuited
– Check and, if required, replace the
sensor
– Replace the wiring harness
F.785 Fan unit 2: Fan blocked – There is no confirmation signal stat-
ing that the fan is rotating
– Check the air route and, if required,
remove any blockages
F.788 Building circuit: Pump fault – The electronics system of the high-
efficiency pump has detected a
fault (e.g. dry running, blockage,
overvoltage, undervoltage) and has
switched off and locked the pump.
– Switch the heat pump off for at least
30 seconds (no current)
– Check the quality of the plug contact
on the PCB
– Check that the pump functions cor-
rectly
– Purging the building circuit
– Check that the dirt filter that is in
place is permeable
F.817 Freq. converter fault: Compressor – Defect in the compressor (e.g. short
circuit)
– Defect in the frequency converter
– Connection cable to the compressor
is defective or loose
– Measure the winding resistance in
the compressor
– Measure the frequency converter
output between the three phases, (it
must be > 1 kΩ)
– Check the wiring harness and plug
connections
F.818 Freq. converter fault: Mains
voltage
– Incorrect mains voltage for operating
the frequency converter
– Shutdown via the energy supply
company
– Measure and, if required, correct the
mains voltage
The mains voltage must be between
195 V and 253 V.
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