Lennox SLP98UHV Error Code Low Pressure Switch Failed Open: Low Pressure Switch Failed Open

What Does Code Low Pressure Switch Failed Open Mean?

Error code E223 on the Lennox IFC control board means the low-stage pressure switch failed to close during the furnace startup sequence. Before any Lennox furnace lights its burners, the combustion air inducer motor must spin up and create a draft through the heat exchanger and exhaust vent. The pressure switch is a safety device that confirms this draft exists — it's connected to the inducer housing via a small rubber hose, and it closes (makes contact) when it senses adequate negative pressure.

When the low-stage pressure switch doesn't close, it means the inducer is not generating enough vacuum. The most common reason is a blockage in the exhaust vent pipe or the combustion air intake pipe. Birds, wasps, ice, snow, leaves, or debris can obstruct these pipes where they exit the home. A less common cause is a cracked or disconnected pressure switch hose, condensate drain blockage (on high-efficiency 90%+ Lennox models), or a weak inducer motor.

The furnace will not ignite with this code present — it's a pre-ignition safety check. On two-stage Lennox models, E223 specifically refers to the low-fire (first stage) pressure switch. If the furnace was running in high fire, you would see E225 instead.

How to Fix It: Check Exhaust Vent and Combustion Air Intake

What You'll Need

• Flashlight
• Step ladder (if vent terminations are above ground level)
• Mirror or phone camera (to look inside vent pipes)

Steps

1. Turn off power AND shut off gas supply Switch the furnace power off at the disconnect switch near the unit AND at the circuit breaker. Locate the gas shutoff valve on the gas line leading to the furnace and turn it to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave the area immediately and call your gas company from outside.
2. Locate the exhaust vent and combustion air intake pipes Find where the exhaust vent and combustion air intake pipes exit your home. On high-efficiency Lennox furnaces (90%+ AFUE like the SLP98 or EL296), these are PVC or CPVC pipes that typically exit through a side wall. There are usually two pipes — one for exhaust (connected to the inducer) and one for fresh air intake. On standard-efficiency models, the exhaust is a metal flue pipe that goes up through the roof.
3. Inspect the vent terminations for blockages Check both pipe openings for any obstructions. Common blockages include: bird nests, wasp nests or mud dauber nests, ice or snow buildup (in winter), leaves or debris, or a screen that has become clogged. Use a flashlight to look inside the pipes as far as you can see. Remove any visible obstructions by hand or with a long stick.
4. Check the vent pipes for proper slope and connections Trace the vent pipes from the furnace to where they exit the home. Check that all pipe joints are intact and sealed — a disconnected joint can prevent the inducer from building proper pressure. On Lennox high-efficiency models, the exhaust pipe should slope slightly downward toward the furnace to allow condensate to drain back.
5. Check the condensate drain (high-efficiency models only) If you have a high-efficiency Lennox furnace (90%+ AFUE), check the condensate drain line and trap. If the drain is clogged, condensate water can back up into the inducer housing and prevent the pressure switch from sensing proper vacuum. Look for water pooled around the furnace base. You can pour a small amount of warm water through the trap to verify it drains freely.
6. Inspect the pressure switch hose With the access panel off, locate the small rubber or silicone hose connected from the inducer housing to the pressure switch. Check that the hose is not cracked, kinked, disconnected, or full of water. If the hose has water in it, gently disconnect one end and drain it, then reconnect.
7. Restore power and test Replace the access panel. Turn the gas supply back on (valve handle parallel to the pipe). Restore power at the disconnect switch and breaker. Set your thermostat to call for heat and listen for the inducer motor to start, followed by the click of the pressure switch closing and the ignition sequence beginning.

When to Call a Professional

Contact a licensed HVAC technician if:

• The exhaust vent and intake are clear but E223 continues to appear
• The inducer motor sounds weak, doesn't start, or makes unusual noises
• You can hear the inducer running but the pressure switch never clicks closed
• There is significant condensate leaking from the furnace or inducer assembly
• You notice a gas smell at any point during the process