C1
FAULT CODE
Viessmann
Differential Air Pressure Sensor Open Circuit
High Severity
Engineer Required
The differential air pressure sensor has an open circuit, indicating a fault with the sensor or its wiring. This sensor confirms correct fan operation before ignition.
Step-by-Step Fix Guide
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Step 1
Turn off the boiler
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Step 2 Engineer only
Call a Gas Safe engineer to test the air pressure sensor and fan system
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.