Low stage pressure switch contacts not closed

What is going on

The control is looking for the low stage pressure switch to close and it isn't seeing it. That switch tells the control the inducer is pulling a draft, so without it the furnace won't go on to light. Usually it's the switch, its hose, or something in the wiring.

Likely causes

• Low stage pressure switch contacts sticking
• A short in the pressure switch circuit wiring
• A cracked, pinched or plugged pressure switch hose

Check in this order

1. Inspect the pressure switch hose and repair or replace it if it is cracked, kinked or has water in it.
2. Look over the pressure switch wiring with the power off for rubbed insulation or shorts.
3. Repair the short in the wiring if you find one.
4. Watch the switch during an inducer start and see whether it closes.
5. Replace the low stage pressure switch if the contacts stick.
6. Do not jumper the switch. It is a safety device.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)