F.68 Burner in a fault state Flame signal is already present at burner start
The F68 error code on Viessmann boilers indicates f.68 burner in a fault state flame signal is already present at burner start.
Viessmann F68 means: f.68 burner in a fault state flame signal is already present at burner start. The usual causes are flame signal is already, present at burner start or close the gas shut-off valve. re. You can safely reset the boiler once — if F68 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does F68 Mean
The F68 code is shown by the boiler’s control board when it detects f.68 burner in a fault state flame signal is already present at burner start. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F68 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 4 technical sources.
| Cause |
|---|
| Flame signal is already |
| present at burner start. |
| Close the gas shut-off valve. Re |
| move connecting cable of the ioni |
| sation electrode. Reset the appli |
| ance. |
Safety Warning
F68 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately. Flame signal is already
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Check the gas supply — verify the gas meter is on and other gas appliances in the property work. 2. Check the gas isolation valve at the boiler is fully open. 3. Measure the inlet gas pressure at the boiler test point — should be 20 mbar (8.0 in wc) for natural gas (UK). 4. During ignition, measure the voltage at the gas valve coil terminals — the PCB should send a signal. 5. If the valve receives voltage but does not open, the gas valve is faulty. 6. Check the gas rate at the meter against the manufacturer's data
How to Fix It
Restore the gas supply if interrupted
replace the gas valve if it is stuck or electrically faulty — Gas Safe registered engineer only
adjust gas pressure if out of specification
repair wiring between gas valve and PCB
if PCB is not sending signal, replace the PCB. Warning: never attempt gas work unless you are Gas Safe registered
Refer to the Viessmann service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Viessmann models that can display fault code F68:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Viessmann models | — | Possible |
Frequently Asked Questions
Restore the gas supply if interrupted. According to the technical documentation, this is the first check on the list for F68.
F68 is not classified as a high-severity fault, so the boiler can be reset once. If F68 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.