Furnace Pressure Switch Error: What It Means

Short answer

The pressure switch confirms that the inducer (draft) fan is pulling combustion gases through the furnace and vent before the burners light. A pressure switch code means the switch was open when it should be closed, or closed when it should be open. Causes listed in the manuals include blocked vents, an inducer not running to speed, blocked or disconnected tubing, and condensate problems.

Pressure switch codes by brand

Code	Meaning
Goodman EE1	Pressure switch circuit closed at start of heating cycle
Goodman EE2	Pressure switch circuit not closed
Carrier 23	Pressure switch did not open
Carrier 31	Low or high pressure switch did not close or reopened
Trane E3.1 / E3.2	1st stage pressure switch shorted / open
Lennox red off / green slow	Pressure prove switch open or blocked inlet/exhaust vent
Rheem 44 / 45 / 46	Low pressure switch closed with inducer off / open with inducer on

Causes listed by manufacturers

Blocked or restricted vent or intake pipe (Rheem FAQ, Lennox, Carrier)
Inducer (draft) blower not running to speed (Rheem FAQ)
Disconnected or obstructed pressure tubing (Carrier)
Water or condensate problems in the collector box (see Rheem 59)
A damaged heat exchanger (Rheem FAQ)

Safe checks

Look at outdoor intake and exhaust terminations for snow, ice, leaves or nests.
Note the code before power is removed.

Do not jumper or bypass a pressure switch. It protects against venting failures that can put carbon monoxide in the home.

Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.

§FFAQ

Can I replace the pressure switch myself?

A failed switch is only one possible cause; blocked vents, inducer or condensate problems are also common. A technician should test it before any part is replaced.

Why is bypassing a pressure switch dangerous?

The switch proves the venting is working. Without it, the furnace could run with a blocked vent and spill combustion gases, including carbon monoxide, into the home.