Low Water Pressure
System water pressure has dropped below the safe minimum level. This is one of the most common Vaillant fault codes and is usually a straightforward fix.
If pressure drops frequently in winter, this is often normal as the system contracts in cold temperatures.
Step-by-Step Fix Guide
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Step 1
Locate the pressure gauge on the front of the boiler — it should read between 1.0 and 1.5 bar
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Step 2
Find the filling loop (usually a grey or silver flexible hose under the boiler with two small valves)
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Step 3
Turn off the boiler
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Step 4
Slowly open both filling loop valves until pressure reaches 1.2 bar
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Step 5
Close both valves fully
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Step 6
Switch the boiler back on and press the reset button
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Step 7
If the pressure drops again within days, there may be a leak — 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.