Five-minute lockout (low pressure switch fails to close)

What is going on

A five-minute soft lockout starts if the low pressure switch does not close within 60 seconds of continuous inducer operation at the start of a heat cycle. The display shows the matching fault. The lockout clears automatically after five minutes, so the furnace may seem to fix itself and then do it again. Be careful when you inspect the vent, because flue gases can contain carbon monoxide. Power off before touching hoses or wiring, and call a licensed technician if the vent or combustion looks wrong.

Likely causes

• Low pressure switch not closing during the proving period
• Blocked or restricted venting, including a clogged intake or terminal
• Loose or faulty wiring
• Faulty pressure switch or hoses
• Condensate backing up into the pressure switch hose

Check in this order

1. Check the vent system for blockage and proper termination
2. Check the pressure switch hoses for blockage, splits, or disconnection
3. Look for water in the hose or switch, which means a drain problem
4. Check the pressure switch and wiring
5. Wait five minutes for automatic reset, or clear the original fault and cycle power
6. Watch the next heat cycle and confirm the switch closes within the proving time
7. If it keeps locking out, have a licensed tech check vent and inducer

More furnace guides

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• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Canada only: derate high altitude by lowering manifold pressure
• Cement PVC vent joints