Viessmann FB2B Installation And Service Instructions Manual page 106

Troubleshooting
Fault codes
(cont.)
Displayed System characteris-
fault code tics
A4 Control mode
A7 Control mode as per
delivered condition
A8 Burner blocked. The
venting program is
started automatically
(see page 66)
A9 The burner operates
at its lower output if a
heating circuit with
mixer is connected.
The burner is blocked
if only one heating cir-
cuit without mixer is
connected.
b0 Burner blocked
b1 Control mode as per
delivered condition
b4 Regulates as if the
outside temperature
was 0 °C
b5 Control mode as per
delivered condition
b7 Burner blocked
106
Cause Measures
Max. system pres- Check system pressure.
sure exceeded Check the function and
sizing of the diaphragm
expansion vessel.
Vent the heating system.
Coding address "0E"
(group boiler/1) is set to
"1" to document the fault.
After the fault has been
removed, reset manually
to "0".
Programming unit Replace programming
faulty unit
Air lock in the inter- Vent the system if the fault
nal circulation message continues to be
pump or minimum displayed
flow rate not ach-
ieved
Internal circulation Check the circulation
pump blocked pump
Short circuit, flue Check the flue gas tem-
gas temperature perature sensor
sensor
Communication Check connections and
error, program- replace the programming
ming unit unit if required
Internal fault Replace the control unit
Internal fault Replace the control unit
Boiler coding card Plug in boiler coding card
faulty or replace, if faulty