121
Control unit
message
System characteristics
Cause
Measures
F.305/C2F2
No power generation
Fault, stack voltage 3 (fuel
cell)
■
Check balanced flue system for:
–
Flue gas recirculation
–
Leaks
■
Check ventilation air supply for:
–
Ingress of flue gas due to non-
compliance with chimney clear-
ances, e.g. by a solid fuel boil-
er
–
Outgassing/contamination in
the air supply system (e.g. sili-
cones, oils, paints, detergents,
deposits, etc.)
If faults recur, contact Viessmann
Technical Service.
Note
Fault message can
only
be reset
on the programming unit of the fuel
cell module.
F.341
Fuel cell in emergency mode
Communication error, fuel
cell
Check the connecting cable to the
fuel cell.
F.342
Burner in a fault state
Communication error,
burner control unit
Check connecting lead to the burn-
er control unit (BCU).
Reset device.
F.343
Fuel cell only heats the cylin-
der module.
Short circuit, common re-
turn temperature sensor
Check return temperature sensor.
F.344
Fuel cell only heats the cylin-
der module.
Lead break, common re-
turn temperature sensor
Check return temperature sensor.
F.345
Burner locked out
Temperature limiter has
responded.
Ensure adequate heat transfer.
F.346
Burner in a fault state
Ionisation current calibra-
tion error
■
Check the gas supply pressure.
Check input-side screen on the
gas train for contamination.
■
Check ionisation electrode for
contamination.
Check the distance of the ionisa-
tion electrode for burner gauze
assembly (see page 79).
Reset device.
■
Check flue system or flue gas re-
circulation.
■
Check connecting cable on fan
unit.
■
Check impeller for ease of opera-
tion.
Reset device.
F.348
Burner in a fault state
Gas modulation valve
Check connecting cable for gas
train for damage.
Check flue system for flue gas con-
gestion.
Check gas train.
Reset device.
Troubleshooting
Fault messages on the gas condensing…
(cont.)
5853335
Diagnosis
