There are about 25 different Vaillant Combination boilers in high circulation which all differ... So you need to be more specific.
On most of the newer appliances such as the TurboMax and ThermoCompact ranges, F28 is ignition lockout.
This can be caused by no gas, low gas pressure at the gas meter's govenor or defective ignition electrodes or leads.
ccMods
July 2006
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