Lennox SLP98UHV Error Code Soft Lockout - No Flame Sensed: Soft Lockout - No Flame Sensed

What Does Code Soft Lockout - No Flame Sensed Mean?

Error code E270 on the Lennox IFC control board indicates a soft lockout condition triggered because the furnace attempted to ignite but no flame was detected after multiple tries. The control board goes through its normal ignition sequence — turning on the inducer motor, opening the gas valve, and energizing the igniter — but the flame sensor never sends back a signal confirming combustion. After the preset number of retries, the board enters soft lockout.

Soft lockout is a temporary safety state. Unlike a hard lockout, the furnace will automatically attempt to restart after a waiting period (typically one hour on Lennox IFC boards, or sooner if power is cycled). However, if the underlying cause isn't fixed, the furnace will just lock out again each time.

The most frequent cause of E270 is a dirty flame sensor — the burners are actually lighting, but the sensor rod is too oxidized to detect the flame and the board shuts the gas off within a few seconds, interpreting it as "no flame." Other possible causes include a cracked igniter that can't light the gas, a gas supply issue, or a faulty gas valve, but a dirty flame sensor accounts for the vast majority of E270 occurrences on Lennox furnaces.

How to Fix It: Clean the Flame Sensor

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

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. 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.
3. 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.
4. 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.
5. 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.
6. 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.
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. Cycling the power will also clear the soft lockout, allowing the furnace to attempt ignition immediately. Set your thermostat to call for heat and observe the startup sequence.

When to Call a Professional

Contact a licensed HVAC technician if:

• Cleaning the flame sensor does not resolve the E270 code
• You can hear the igniter click but never see burner flames ignite
• The burner flames ignite but appear yellow, orange, or irregular
• You notice a gas smell at any point during the process
• The furnace cycles between E270 and other error codes such as E228 or E200