How to Fix a Viessmann Vitodens 100 FF Fault Code

What Does the Viessmann Vitodens 100 FF Fault Code Mean?

The FF fault code means the boiler attempted to ignite but failed to detect a stable flame. As a safety measure, the boiler locks out to prevent unsafe operation until the fault is resolved.

• Flame detection failed.
• Boiler entered safety lockout.
• Further diagnosis is required if the fault returns after resetting.

If you're seeing a different error, our guide to Viessmann boiler fault codes explains what each code means and the checks you can safely carry out.

Common Causes of the FF Fault Code

Common causes include:

Low Gas Supply

Reduced or interrupted gas supply can prevent reliable ignition.

Burner or Flue Blockage

A blocked burner or flue can disrupt combustion and stop the flame sensor detecting a stable flame.

Faulty Ionisation Electrode

The ionisation electrode confirms the flame is present. A dirty or damaged electrode can misread the flame as absent.

Frozen or Blocked Condensate Pipe

A frozen condensate pipe can cause the boiler to lock out, which can present alongside or instead of a genuine ignition fault.

Low System Pressure

Insufficient system pressure can prevent the boiler from firing correctly.

Electrical or Control Faults

Faults in the control system or PCB can interrupt the ignition sequence even when the burner and gas supply are working normally.

How to Troubleshoot the Viessmann Vitodens 100 FF Fault Code

Step 1 — Check the Gas Supply

Confirm other gas appliances are working and that any prepayment meter has sufficient credit.

Step 2 — Check Boiler Pressure

Check the pressure gauge and repressurise using the filling loop if the reading is low.

Step 3 — Inspect the Flue

Check the flue terminal outside the property for obvious blockages or damage.

Step 4 — Check the Condensate Pipe

Look for signs of freezing or blockage, particularly in cold weather.

Step 5 — Inspect the Ionisation Electrode

Internal burner cleaning should only be carried out by a Gas Safe registered engineer.

Step 6 — Reset the Boiler

Once you've completed the checks above, reset the boiler and allow the ignition cycle to complete.

Step 7 — Call a Gas Safe Engineer

If the electrode needs testing or replacing, arrange a professional repair

Can You Prevent the FF Fault Code?

Annual servicing helps reduce unexpected breakdowns by identifying worn or faulty components before they fail. It also keeps the burner, electrodes and safety devices working correctly, making recurring ignition faults less likely.

When to Book a Boiler Repair

When professional repair is needed, experienced Gas Safe heating engineers can identify and fix the underlying cause safely — rather than repeatedly resetting a boiler that keeps losing ignition. If the FF fault keeps returning after these checks, arrange a professional Viessmann boiler repair to diagnose the underlying cause safely. A safe repair completed correctly the first time is the priority when a recurring ignition fault is involved.

Frequently Asked Questions

Yes, you can reset the boiler once you've completed the basic checks above. If the fault returns immediately, stop resetting it and book an engineer.

The lockout itself is a safety feature, not a fault in progress — but a persistent ignition problem should be diagnosed by a Gas Safe engineer rather than repeatedly reset.

Yes. Reduced gas pressure can prevent the burner igniting reliably, which the boiler reads as a flame-detection failure.

Need Help With an FF Fault Code?

If your Viessmann boiler still shows an FF fault after resetting and carrying out the basic checks, our Gas Safe engineers can diagnose the problem and carry out the repair safely. Book a boiler repair with Serviceteam today.