Lennox Furnace Error Code DS1/DS2 Alternating Fast Flash

This flash pattern indicates a problem detected during the ignitor warm-up phase of the furnace startup sequence. The three possible causes are an improper main ground, a broken or open ignitor circuit, or supply line voltage that has dropped below 75 volts. The fault is specific to the warm-up window, which helps narrow down the diagnosis quickly. Checking the ground connection, measuring ignitor resistance, and verifying line voltage are the first steps.

The fault itself is a safety response. The control board detected that ignition conditions were not correct and halted the startup sequence before gas was released. As long as the furnace has locked out and is not attempting repeated ignition cycles, there is no immediate danger. Do not attempt to bypass the lockout or defeat the fault detection system.

Resetting the furnace may temporarily clear the fault and allow the board to attempt another startup cycle. However, if the underlying cause, such as a failed ignitor, bad ground, or low voltage, has not been corrected, the furnace will fault again during the next warm-up attempt. Repeated reset attempts without addressing the root cause can lead to additional wear on components and will not result in a successful heat call.

Some causes are within reach of a knowledgeable DIYer. Checking and tightening the ground connection or inspecting the wiring harness for visible damage requires only basic tools. Measuring ignitor resistance with a multimeter is also straightforward if you are comfortable working around furnace components with the power disconnected. However, if the fault is caused by low line voltage or a wiring issue upstream of the furnace, a licensed electrician should handle that portion of the repair.

Repair cost depends heavily on the root cause. If the issue is a loose ground connection, the cost may be limited to a service call fee, typically in the range of $75 to $150. Ignitor replacement parts generally cost between $30 and $80 for the component alone, with total repair costs including labor often falling in the $150 to $300 range. Low line voltage issues traced to household wiring or the electrical panel can be more expensive to resolve depending on the scope of the electrical work needed.