Display shows code 3, furnace does not start, fan may run briefly

Safety first: Isolate power. Do not operate the boiler with a known flue or combustion air fault.

Basic external checks:

• Inspect flue terminal: From outside, ensure the Intergas concentric flue terminal is clear of obstructions (ice, leaves, nests). Remove any blockage without damaging the terminal.
• Check air intake grills: Make sure any air intake openings are not covered or restricted.

Technician-level checks:

• Inspect fan operation: With the cover removed and power restored, observe whether the fan starts when there is a call for heat. If the fan does not run or is noisy, test supply voltage and replace the fan assembly if defective.
• Check air pressure switch: Inspect silicone tubes from the fan/venturi to the pressure switch for splits, kinks, or water. Replace tubing if damaged.
• Test pressure switch contacts: Using a multimeter, verify that the air pressure switch changes state when the fan runs. Replace the switch if it fails to operate reliably.
• Check flue system: Inspect the full length of the flue for sagging, disconnections, or internal blockage. Repair or replace as per Intergas flue installation guidelines.

Because combustion safety is involved, any persistent code 3 fault should be handled by a qualified gas technician.