Lennox Furnace Error Code 4+2

The 4+2 blink code on a Lennox furnace with control board 100869 indicates a Watchguard lockout. The furnace either exceeded its maximum number of ignition retries or the pressure switch opened during the last retry attempt. This is a protective shutdown to prevent unsafe operation, and the root cause must be diagnosed and corrected before the furnace will run normally.

Yes, the furnace has shut down because it could not confirm safe operating conditions during ignition. Running a furnace that repeatedly fails its proving sequence could result in unburned gas accumulation or improper venting. The Watchguard lockout is working as intended by stopping operation until the problem is resolved, so the furnace should not be forced to run until the fault is properly diagnosed.

Repair costs vary widely depending on the root cause. Clearing a blocked vent termination may involve only a service call fee, typically in the range of $75 to $150. Replacing a pressure switch hose or the switch itself generally costs between $100 and $300 including parts and labor. An inducer motor replacement is a more significant repair, often ranging from $300 to $700 or more depending on the specific motor and local labor rates. A technician's diagnosis will clarify which component is responsible before any parts are ordered.

Some causes, such as a blocked vent pipe termination or a disconnected pressure switch hose, can be addressed by a knowledgeable homeowner. However, testing the pressure switch with a manometer, evaluating the inducer motor, and diagnosing the ignition system typically require tools and experience that most homeowners do not have. If you are not comfortable working around gas appliances and electrical components, contacting a licensed HVAC technician is the safer choice.

Resetting the furnace by cycling the power may temporarily clear the lockout and allow the furnace to attempt ignition again. However, if the underlying cause, such as a blocked vent, a failed pressure switch, or a weak inducer motor, is not corrected, the furnace will go through its retry sequence and lock out again with the same 4+2 code. A reset is useful for confirming the fault is consistent, but it is not a repair.