V

Check the gas supply to the property and the inlet on the gas valve (check drop on inlet).
Check the connections on the gas valve and the board (loose connection).
Check the continuity down harness and replace if required.
Check the voltage to the gas valve (19–24 V DC approx. when demand).
Check for water on the gas valve connections.
Check the resistance on the gas valve coils pins 1–2 (SIT; 55...66.8...85 / Resideo; 110...114.5...120 ).
Replace the gas valve if resistances are incorrect.
If all tests are correct: replace the control unit.