Viessmann Fault Code BC — Diagnose and Fix
Fault code BC on a Viessmann boiler indicates low pressure. System water pressure has dropped below 1.0 bar. The boiler locks out to protect the pump. Repressurise via the filling loop to 1.2 bar cold. Here is the correct diagnostic order UK Gas Safe engineers follow before ordering parts.
Step-by-step method
1. Confirm the BC lockout
Cycle the boiler off at the fused spur for 60 seconds, restore power and press the reset button. If BC returns within 5 minutes, proceed with diagnosis. Intermittent BC usually points to a sensor or connection, not the PCB.
2. Check pressure and condensate first
Every fault-code diagnosis starts here: system pressure 1.0–1.5 bar cold, condensate pipe unfrozen and draining, gas supply proven at another appliance. These three account for ~60% of lockouts.
3. Test the component BC points at
System water pressure has dropped below 1.0 bar. The boiler locks out to protect the pump. Repressurise via the filling loop to 1.2 bar cold.
4. Reset and test-fire
After any component test, purge the gas line, restore all covers (interlocks matter), press reset and observe a full ignition cycle. Confirm flame current with a multimeter on the flame sense wire (typically 1–8 µA DC).
5. Call a Gas Safe engineer for parts
Any gas-side work on a Viessmann requires Gas Safe registration. Have the fault code, boiler GC number and serial to hand — engineers can pre-order the right part before attending.
Frequently asked questions
Can I reset a Viessmann BC myself?+
Yes — press and hold the reset key for 5 seconds. If BC returns, the underlying fault is still present and needs diagnosis.
How much does fixing BC cost?+
£120–£280 typical, depending on part. PCBs are £180–£320 fitted; sensors £60–£100. Get a fixed-price quote — refuse hourly for diagnostic-only work.
Is BC covered by boiler warranty?+
Yes if within the Viessmann manufacturer warranty period AND the boiler has valid annual services logged. Missed services void most warranties.