3 Low pressure switch did not close or reopened

What the control is telling you

Low pressure switch did not prove inducer draft or reopened. If it opens during blower on-delay, blower runs for the off-delay. If it opens within 5 minutes after ignition, the next cycle is limited to maximum heat. Also the expected code when the inducer is unplugged during the pressure-switch safety check.

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

Likely causes

• Wrong vent sizing / restricted vent
• Air leak between vestibule and blower compartment
• Low inlet gas pressure (if LGPS fitted)
• Disconnected or blocked pressure tubing
• Defective or miswired pressure switches
• Excessive wind
• Plugged condensate drain
• Water in vent pipe (sagging pipe)

Test and repair steps

1. Check vent and combustion air pipes for proper size and blockages.
2. Check pressure tubing is connected and clear; clean the collector box pressure port with a small wire.
3. Flush the collector box and clean the condensate trap and drain; check vent pipe for sags holding water.
4. Seal air leaks between the vestibule and blower compartment.
5. If LGPS used, check inlet gas pressure 4.5 to 13.6 in. w.c. natural gas at maximum heat.
6. Check pressure switch wiring.
7. In windy locations, if the problem persists, turn SW1-3 ON to boost inducer speed.