Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.93 is combustion outside the permitted window during the gas-family check, or the check was aborted. Causes include the wrong restrictor, recirculation, wrong family, or a blocked Venturi pressure tapping (do not oil the Venturi O-ring).
Vaillant meaning: Gas group / combustion-quality fault
Possible causes
- Wrong gas restrictor
- Flue-gas recirculation
- Wrong gas family selected
- Blocked internal pressure point in the Venturi
- Gas-family check stopped early
Possible solutions
- Confirm gas type and restrictor against the installation manual.
- Check flue integrity (F.35 / recirculation).
- Inspect the Venturi; do not use lubricant on the O-ring. Clear the pressure tapping.
- Repeat the gas-family check in full and set CO/CO₂ to the emissions table — see ecoTEC emissions table.
Possible parts needed
- Correct gas restrictor for NG or LPG
- Venturi / mass-flow parts if tapping or filter is blocked
- Flue seals if recirculating
- Gas valve only if mixture cannot be set with a sound Venturi and correct restrictor
