Viessmann VITODENS 300 WB3A Service Instructions Manual page 27

Further details regarding the individual steps
Control unit
issues heat
demand
Yes
Fan starts
Yes
Differential pres-
sure sensor
enables start
(0.75 V across
terminals X8.5
and X8.6)
Yes
Ignition
Yes
Gas combination
valve opens
Yes
Initial start-up, inspection, maintenance
Display screen
No
After approx.
51 seconds
No
Fault
F9
After approx.
51 seconds
No
Fault
E6
Fault
No
F4
Fault
No
F4
(cont.)
Remedy
Set value
Check cables and
plug-in connec-
tions
Check fan cables
and fan plug-in
connections,
check fan
Check the ignition
module (control
voltage 230 V
across plugs X2.1
and X2.2)
Check the gas
train (control vol-
tage 230 V) and
gas supply pres-
sure
11