Lennox Furnace Error Code DS1 Slow Flash / DS2 ON

This code means that one or more limit switches, including the primary, secondary, backup secondary, or rollout limit switch, has opened during operation. These switches are safety devices that trip when they detect overheating or flame rollout. If the switch does not close again within 3 minutes, the furnace enters a 1-hour Watchguard lockout and suspends heating. The root cause is usually restricted airflow or a faulty limit switch.

Resetting the furnace can clear the 1-hour Watchguard lockout and allow the unit to attempt another heating cycle, but it will not fix the underlying problem. If the cause of the limit switch opening is not corrected, the fault will return and the lockout will activate again. Identifying and resolving the root cause, such as restricted airflow or a failed switch, is necessary for a permanent fix.

If the fix is simply replacing a clogged air filter, the cost is minimal, typically a few dollars to around twenty dollars for a filter. If a limit switch or rollout switch needs replacement, parts and labor generally range from around one hundred fifty dollars to three hundred fifty dollars depending on the switch type and local labor rates. A failed blower motor is a more significant repair and can range from three hundred dollars to over six hundred dollars including labor. Getting a diagnosis from a qualified technician will clarify which component is at fault.

Start by checking and replacing the air filter, then inspect the blower inlet and all supply and return registers for obstructions. Use a multimeter to check continuity across each limit switch and rollout switch to identify any that are open and not resetting. If airflow is adequate and the switches still will not close, one or more switches may need to be replaced. Also inspect the flue and venting system for blockages that could cause abnormal heat buildup.

The furnace is designed to shut down when this code is active, which is the safe behavior. You should not attempt to bypass or defeat the limit switches, as they protect the heat exchanger and the home from overheating and potential fire hazards. Flame rollout in particular can indicate a serious combustion problem that requires inspection before the furnace is returned to service.