Viessmann Vitodens 300-W Service Manual page 84

Troubleshooting
Repairs
(cont.)
Checking the flue gas temperature sensor
The flue gas temperature sensor locks out the boiler when the permissible flue
gas temperature is exceeded. Reset the interlock after the flue gas system has
cooled down by pressing "E RESET".
84
1. Boiler water temperature sensor:
Pull the leads from boiler water
temperature sensor A and
check the resistance.
Cylinder temperature sensor: Pull
plug % from the cable harness at
the control unit and check the
resistance.
Flow temperature sensor: Pull
plug "X3" from the control unit
and check the resistance across
terminals "X3.4" and "X3.5".
2. Check the sensor resistance and
compare the actual values with the
curve.
3. Replace the sensor in case of
severe deviation.
Danger
The boiler water tempera-
ture sensor is immersed in
the heating water (risk of
scalding).
Drain the boiler before repla-
cing the sensor.