Furnace pressure switch proves combustion-air and vent pressure before ignition. Related symptoms include inducer without ignition, pressure code or lockout. Those symptoms do not prove the component failed: inspect only exterior intake/exhaust blockage and condensate. Look at the outside vent and intake terminations and clear only what you can reach safely from outside.
What this covers
- induced-draft gas furnaces
What it does not cover
Differential
Likely causes of “Furnace pressure switch”, and how to tell them apart
- 1
The exhaust or intake pipe outside is blocked
What separates it The inducer fan starts and the furnace stops with a pressure fault, and snow, a bird nest or leaves are covering the vent or air-intake pipe outside.
How to confirm Look at the outside vent and intake terminations and clear only what you can reach safely from outside.
- 2
Condensate or a blocked drain is affecting the pressure line
What separates it A high-efficiency furnace stops with a pressure code, water is visible in the trap or the drain line is clogged.
How to confirm Look at the condensate trap and drain tubing for water or blockage without removing anything.
- 3
The vent run is damaged, disconnected or has a new obstruction
What separates it The fault began after work on the house, a storm or a new appliance was connected to the same vent.
How to confirm Ask what changed and have the vent run inspected by a qualified person.
- 4
The pressure switch or its tube has failed
What separates it The inducer runs, the vent and drain are clear and the furnace still shows a pressure fault.
How to confirm This needs a technician with a manometer to test the switch and its tubing.
Before you touch a setting
Record this first — changing settings destroys the evidence
- The furnace's blink code or displayed fault and how far into the start-up sequence it happens.
- Whether the inducer fan runs and whether the burners ever light.
- The state of the outdoor vent and intake and of the condensate trap.
Try in this order
0 of 2 completedTest before you replace
ReversibleInspect only exterior intake/exhaust blockage and condensate.
Weigh the repair
ReversibleReplacement difficulty for the furnace pressure switch is professional; never jumper a pressure switch.
Stop boundary
When this stops being a self-service repair
- !Do not bypass or jump a pressure switch; it exists to stop the furnace from running with a blocked or leaking vent, which can produce carbon monoxide.
- !Leave the house and call the gas utility or emergency services if a carbon monoxide alarm sounds or you smell gas.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Is a pressure switch fault a dangerous one?+
The fault is the furnace protecting you from unsafe venting. Treat it seriously, check outdoor terminations and let a technician test it rather than bypassing it.
What does the pressure switch do?+
It confirms the draft inducer is moving air before the burners light. If it does not close, the furnace will not fire.
What causes it to fail?+
A blocked vent, a bad inducer, a cracked tube or a failed switch.
Should I bypass it?+
Never. It is a safety device, so a technician should diagnose the cause.