Quote from Service manual (available as pdf from vailliant site)
Fault 28 - Fault in the gas supply such as:-Gas meter or Gas pressure detector defective. Air in gas, gas pressure too low. Fire detection tap has disengaged. Fault in the gas valve, wrong gas setting, Igniter (ignition transformer, ignition cable, ignition plug) defective. Ionisation currnent stopped (cable, electrode), Faulty earthing in appliance, electronics defective.
S98Flow/Return sensor check is running. demand (DHW or Heating) is blocked.
Peter kay, December 2007
