4 Limit circuit fault

What the control is telling you

Main limit or a flame rollout switch is open, or furnace is locked to maximum heat after 2 limit trips in a row while firing below 71% of maximum. Blower runs 4 min or until switch recloses. Open more than 3 min becomes code 7.

Applies to: ICP F9MAE / G9MAE modulating variable-speed condensing gas furnaces (060, 080, 100, 120 kBtu/h)

Likely causes

• Dirty filter or restricted ducts
• Loose blower wheel
• Defective limit/rollout switch or connections
• Wrong minimum/maximum heat gas input setting
• Wrong limit switch or missing limit gasket
• Flame rollout switch tripped (manual reset)

Test and repair steps

1. Warning: A tripped flame rollout switch means flames came out of the burner box - find the cause (blocked heat exchanger/vent) before resetting.
2. Replace dirty filters and check supply/return ducts for restrictions (static over 0.5 in. w.c. is too high).
3. Check the blower wheel is tight and the blower runs.
4. Check limit and rollout switches and wiring for continuity; reset the rollout switch manually.
5. Check manifold pressure at maximum and minimum heat and correct gas input.
6. Check temperature rise is in the rated range.
7. Verify the correct limit switch and gasket are installed.