Pump / Pressure Sensor Fault
The boiler cannot detect a pressure change when the pump starts. This is the most common Vaillant fault code, typically indicating a faulty pump, airlocked system, or failed pressure sensor.
Step-by-Step Fix Guide
-
Step 1
Check the system pressure gauge — it should read 1.0 to 1.5 bar
-
Step 2
If low, repressurise using the filling loop
-
Step 3
Bleed all radiators to remove airlocks, starting from the ground floor
-
Step 4
Reset the boiler
-
Step 5
Listen for the pump running (a quiet hum from inside the boiler casing)
-
Step 6
If pressure is correct and pump sounds normal but fault returns, the pressure sensor needs replacing — call a Gas Safe engineer
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.