Lennox Furnace Error Code LED Flashing 1/2 sec on 1/2 sec off

This flash pattern indicates a Lockout or Restricted Lockout condition on the R7184B oil primary burner control. It means the control attempted to start the oil burner, could not confirm a safe flame was established, and shut the system down as a safety measure. The burner will not restart until the fault is identified and the control is manually reset. Repeated lockouts without a successful restart suggest a Restricted Lockout, which requires resolving the root cause before the reset will hold.

The lockout itself is a safety feature, not a hazard. The R7184B shuts the burner down specifically to prevent unburned oil from accumulating in the firebox, which could create a dangerous condition if ignition were attempted again. However, repeatedly pressing the reset button without diagnosing the problem can introduce unburned fuel into the combustion chamber, so limit manual resets and call a technician if the issue does not resolve.

The cad cell is the flame sensor for oil burners and changes resistance based on light exposure. If it detects light before the ignition sequence starts, the R7184B assumes a flame already exists and goes into lockout. First check that no external light is reaching the cell by inspecting its mounting position. Next, clean the face of the cad cell and test its resistance in darkness, comparing it to specification. A cad cell that reads low resistance in the dark or does not respond correctly to the burner flame should be replaced.

You can press the reset button on the oil primary control to attempt a restart, but resetting without fixing the underlying problem will typically result in another lockout within the same ignition cycle. If the furnace locks out more than once, avoid repeated reset attempts, as this can flood the combustion chamber with unburned oil. Identify and correct the cause before resetting again.

Repair costs depend on the root cause. A cad cell replacement is a relatively inexpensive fix, typically in the range of $80 to $200 including labor. Replacing the oil nozzle or igniter assembly adds moderate parts cost. If the R7184B oil primary control itself has failed, parts and labor can run $200 to $450 or more depending on your region and service provider. A diagnostic service call is usually the first cost, ranging from $75 to $150.