Lennox Furnace Error Code DS1/DS2 Alternating Slow Flash

This fault code indicates the furnace control board has activated a Watchguard lockout due to one of four conditions: the burners failed to ignite, the high-limit switch stayed open for more than 3 minutes, the flame signal was lost 5 times in one heating cycle, or the pressure switch opened 5 times in one heating cycle. The furnace suspends operation to prevent damage or unsafe repeated ignition attempts. Diagnosis is required to identify which of the four conditions triggered the lockout.

The Watchguard lockout is itself a safety feature, meaning the furnace has already taken a protective action by shutting down. The concern is the underlying cause. A gas supply or ignition problem that prevents proper burner operation, or an airflow restriction causing the heat exchanger to overheat, can lead to more serious issues if left unresolved. Have the furnace inspected and repaired before returning it to regular operation.

Repair costs vary widely based on the root cause. Cleaning a flame sensor is typically a low-cost service call. Replacing a hot surface ignitor generally runs in the range of $150 to $350 including labor. A failed inducer motor or control board replacement can cost significantly more, often in the $400 to $900 range or higher depending on the model and local labor rates. A technician diagnosis is needed to provide an accurate estimate.

A power reset may temporarily clear the lockout and allow the furnace to attempt another start cycle. However, if the underlying problem is not corrected, the furnace will detect the same fault condition and return to lockout. Resetting without diagnosing and repairing the root cause typically results in repeated lockouts and should be considered a temporary measure only.

Start by checking the basics: confirm gas supply is on, inspect the air filter and replace it if dirty, and look for obvious vent blockages. If those checks pass, a qualified technician should test the hot surface ignitor, clean or replace the flame sensor, check the high-limit switch, and inspect the inducer motor and pressure switch circuits. The repair path depends on which component is causing the fault.