Error Code Soft Lockout - Pressure Switch Recycle Failure

Lennox SLP98UHV Error Code Soft Lockout - Pressure Switch Recycle Failure: Soft Lockout - Pressure Switch Recycle Failure

TL;DR

Your Lennox furnace ignited successfully but lost the flame too many times during operation, triggering a soft lockout. A dirty flame sensor is the most likely culprit — the sensor loses its ability to detect the flame intermittently as it heats up.

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 Soft Lockout - Pressure Switch Recycle Failure Mean?

Error code E272 on the Lennox IFC control board indicates the furnace successfully lit the burners but then lost the flame signal multiple times during a heating cycle, exceeding the board's retry limit. Unlike E270 (which means flame was never detected at all), E272 specifically means the flame was established and then lost — this happened repeatedly until the board gave up and entered soft lockout.

This pattern of lighting and then losing flame is a classic symptom of a flame sensor that is partially functional. The sensor may detect the flame initially when it's cool, but as carbon buildup on the rod heats up, its conductivity changes and the signal drops below threshold. Alternatively, the flame sensor wire connection may be intermittent — good enough to register sometimes but not reliably.

E272 is a soft lockout, so the furnace will attempt to restart automatically after the timeout period (typically one hour). However, without fixing the root cause, the same cycle of light-lose-retry-lockout will repeat. Cleaning the flame sensor resolves this issue in the vast majority of cases.

How to Fix It: Clean the Flame Sensor

⚠ 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

Fine emery cloth (220-grit) or Scotch-Brite pad
1/4-inch hex head driver or nut driver
Flashlight
Clean dry cloth

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.
Remove the furnace access panel Remove the lower front access panel of your Lennox furnace to expose the burner assembly. On most Lennox models (SLP98, EL296, etc.), the panel lifts up and pulls out or is held by a couple of screws.
Locate the flame sensor The flame sensor is a thin metal rod (usually slightly bent) mounted near the burners, typically on the opposite end from the igniter. It is held in place by a single 1/4-inch hex screw and has one wire connected to it. On Lennox furnaces, it is usually positioned near the right side of the burner assembly.
Remove the flame sensor Disconnect the wire from the flame sensor by pulling the connector straight off — do not yank the wire itself. Then remove the single 1/4-inch hex screw holding the sensor bracket in place. Carefully slide the sensor out of the burner assembly.
Clean the sensor rod Using fine emery cloth (220-grit) or a Scotch-Brite pad, gently rub the metal rod portion of the flame sensor until it is clean and shiny. Do NOT use steel wool, as it can leave metal fibers that cause electrical issues. Focus on the section of the rod that sits inside the flame. Wipe the rod with a clean dry cloth to remove any dust or residue.
Check the wire connection Before reinstalling, inspect the flame sensor wire connector. If it looks corroded, burnt, or loose, clean the metal contact inside the connector with the emery cloth. A bad connection here can cause the intermittent flame loss that triggers E272.
Reinstall the flame sensor Slide the flame sensor back into position, secure it with the 1/4-inch hex screw, and reconnect the wire to the sensor terminal. Make sure the connection is snug and seated fully.
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 observe the startup sequence.

How to Verify

After restoring power and calling for heat, the burners should ignite and stay lit for the entire heating cycle without interruption. Let the furnace run through at least one full cycle (until the thermostat is satisfied and the burners shut off normally). If the flame drops out mid-cycle and the E272 returns, the flame sensor itself may need replacement.

When to Call a Professional

Contact a licensed HVAC technician if:

The flame sensor has been cleaned but the furnace still loses flame mid-cycle
The burner flames appear uneven, lifting off the burners, or changing color
You notice a gas smell at any point during the process
The furnace displays E272 along with pressure switch codes (E223, E225, E227)
The flame sensor wire or connector is damaged and needs replacement