Lennox Furnace Error Code DS1 Slow Flash / DS2 Fast Flash

This error code indicates that the furnace control board is measuring a flame signal below 0.23 microamps from the flame sense rod. The flame sense rod uses a process called flame rectification to confirm the burner is lit. When the signal drops too low, the board triggers this fault to alert a technician. The most common cause is a dirty or oxidized flame sense rod.

A low flame signal fault means the furnace control board has reduced confidence that combustion is being properly confirmed. In most cases the furnace will attempt to shut down the burner when this fault is detected, which is a safety protection. It is not recommended to continue operating the furnace with this fault present without diagnosing and correcting the root cause.

Cycling power to the furnace may temporarily clear the fault and allow the furnace to attempt another ignition sequence, but if the underlying cause is not corrected, the error will return. The flame signal must measure at or above 0.23 microamps during burner operation for the fault to be resolved. Cleaning or replacing the flame sense rod is necessary for a permanent fix.

Cleaning the flame sense rod is a task many experienced DIYers can perform. The rod can be removed and lightly cleaned with steel wool to remove oxidation or carbon buildup. However, if the rod needs replacement, or if the issue is related to gas pressure or burner condition, a qualified HVAC technician should perform the work. Always shut off power and gas to the furnace before beginning any service.

If only cleaning is required, the cost is minimal, typically limited to a service call fee ranging from $75 to $150 depending on your area. If the flame sense rod needs replacement, the part itself is generally inexpensive, ranging from $10 to $40, with total repair costs including labor often falling between $100 and $250. More complex causes such as gas pressure issues or burner problems could increase the cost.