F3 displayed, fan runs but no ignition on power‑vent gas model

Safety first: Turn off power and gas. Do not operate the heater with a suspected venting problem.

• Inspect venting: Check the entire length of the Giant power‑vent intake and exhaust pipes for obstructions, snow, nests, or debris. Clear any blockage.
• Check condensate drain: If the unit is condensing, ensure the condensate trap and drain line are not clogged or frozen.
• Examine pressure tubing: Verify the small rubber tube from the blower housing to the pressure switch is securely connected and not cracked or kinked.
• Test pressure switch: With a multimeter, check continuity across the pressure switch terminals when the blower is running. If it does not close, replace the Giant pressure switch.
• Control diagnosis: If the switch operates correctly but F3 remains, the control board input may be faulty and should be replaced by a technician.