Viessmann Fault Code C1 — Diagnose and Fix
Fault code C1 on a Viessmann boiler indicates low pressure. System pressure below 1.0 bar — repressurise via filling loop to 1.2 bar cold. Here is the diagnostic order UK Gas Safe engineers follow.
Step-by-step method
1. Confirm the C1 lockout
Power off at the fused spur for 60 seconds, restore and reset. If C1 returns within 5 minutes, proceed. Intermittent C1 points to a sensor or connection, not the PCB.
2. Check pressure, condensate, gas
System pressure 1.0–1.5 bar, condensate unfrozen, gas proven at another appliance — accounts for ~60% of lockouts.
3. Test what C1 points at
System pressure below 1.0 bar — repressurise via filling loop to 1.2 bar cold.
4. Reset and prove
Reinstate covers (interlocks matter), reset and observe full ignition. Flame current 1–8 µA DC.
5. Gas Safe callout
Gas-side work on a Viessmann requires Gas Safe. Provide the fault code, GC number and serial so parts are pre-ordered.
Frequently asked questions
Reset Viessmann C1 myself?+
Hold reset 5 seconds. If it returns, fault is still present.
Repair cost?+
£120–£280 typical; PCBs £180–£320; sensors £60–£100.
Warranty cover?+
Yes if within warranty and annual services logged.