Error Code Low Pressure Switch Open During Ignition/Run

Lennox SLP98UHV Error Code Low Pressure Switch Open During Ignition/Run: Low Pressure Switch Open During Ignition/Run

TL;DR

Your Lennox furnace's pressure switch opened unexpectedly during ignition or while the burners were running, indicating a sudden loss of combustion venting. Check the exhaust vent and intake pipes for intermittent blockages like wind backdrafts or partial obstructions.

Disclaimer

This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code Low Pressure Switch Open During Ignition/Run Mean?

Error code E227 on the Lennox IFC control board indicates that a pressure switch that had already closed (proving adequate draft) subsequently opened during the ignition sequence or while the burners were firing. Unlike E223 or E225 where the switch never closed at all, E227 means the switch closed successfully but then lost its seal — the inducer vacuum dropped below threshold during operation.

This is a particularly concerning code because it means combustion venting was interrupted while gas was potentially flowing. The board immediately shuts down the gas valve when this happens to prevent combustion gases from entering the home. The furnace will not restart until the pressure switch closes again.

Common causes include wind gusts backdrafting down the exhaust vent (especially if the vent termination doesn't have a proper wind guard), a partially blocked vent that allows intermittent airflow disruption, condensate water sloshing in the inducer or trap, or an inducer motor that is weakening and cannot maintain consistent vacuum. Ice buildup that partially melts and re-freezes can also cause intermittent blockages in winter.

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

⚠ Safety First

Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

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

Steps

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.
Inspect the exhaust vent and intake terminations outside Find where the exhaust vent and combustion air intake pipes exit your home. Check for partial blockages — debris that could shift in wind, ice buildup that may intermittently block the opening, or a bird nest or screen that is partially clogged. Because E227 is an intermittent failure, look for blockages that could come and go rather than complete obstructions.
Check for wind exposure issues Assess whether the vent terminations are in a location exposed to strong wind gusts. If the exhaust vent exits on a side of the house that faces prevailing winds, gusts can temporarily overpower the inducer and push exhaust back into the vent, causing the pressure switch to open. Note this for a professional if applicable — a wind guard or vent relocation may be needed.
Inspect the vent pipe connections Trace the vent pipes from the furnace to the exterior. Check for any joints that are loose or poorly sealed. A loose joint can intermittently leak, reducing vacuum just enough to cause the pressure switch to open during operation.
Check the condensate drain and trap On high-efficiency Lennox furnaces (90%+ AFUE), inspect the condensate drain and trap. If water is pooling in the inducer housing or trap is partially blocked, the water level can fluctuate during operation and intermittently block the pressure port. Clear any blockages and ensure the drain flows freely.
Check the pressure switch hose Inside the furnace, check the rubber hose from the inducer to the pressure switch. A hose with a tiny crack or a connection that is barely hanging on can lose its seal intermittently — holding vacuum during startup but losing it during operation due to vibration.
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 let the furnace run through a complete heating cycle. Stay nearby to observe whether the furnace completes the cycle or cuts off mid-run.

How to Verify

After restoring power and calling for heat, the furnace should start and run through an entire heating cycle without interruption. Since E227 is often intermittent, monitor the furnace over several heating cycles or over a day or two. If the code only appears during windy conditions, the vent termination may need a professional-installed wind guard.

When to Call a Professional

Contact a licensed HVAC technician if:

The vents are clear and connections are tight but E227 keeps recurring
The code only appears during windy weather, suggesting a vent termination issue
The inducer motor sounds inconsistent — speeding up and slowing down
There is condensate leaking from the inducer assembly
You notice a gas smell at any point during the process