F.20 — Safety switch-off: Temperature limiter triggered

What does F.20 mean?

The safety temperature limiter has activated, causing the boiler to switch off. This typically happens if the boiler detects an unsafe overheating condition.

• Incorrect earth connection between the cable harness and boiler.
• Flow or return NTC sensor is defective or has a loose connection.
• Electrical discharge (black discharge) via the ignition cable, plug, or electrode.

Click steps to track your progress.

1. 1

   Press the fault clearance key on the boiler to reset it.

2. 2

   Visually check for any loose electrical connections or signs of burning around the ignition system.

3. 3

   If the fault persists, contact Vaillant Service Solutions as this indicates a potentially serious electrical or overheating issue.