Vaillant Fault Code F20 — Diagnose and Fix
Fault code F20 on a Vaillant boiler indicates no ignition. Gas valve opens but spark or flame detection fails. Here is the diagnostic order UK Gas Safe engineers follow.
Step-by-step method
1. Confirm the F20 lockout
Power off at the fused spur for 60 seconds, restore and reset. If F20 returns within 5 minutes, proceed. Intermittent F20 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 F20 points at
Gas valve opens but spark or flame detection fails.
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 Vaillant requires Gas Safe. Provide the fault code, GC number and serial so parts are pre-ordered.
Frequently asked questions
Reset Vaillant F20 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.