The furnace runs, sort of, but not right — short cycling, a burner that won't stay lit, a blower that won't shut off.
A flame sensor's job is to prove a flame exists so the control board doesn't keep dumping gas into a burner with nothing burning it. Over a season or two the sensor picks up a layer of oxide that insulates it enough to still detect the flame at first and lose the signal within seconds — the furnace lights, runs briefly, and shuts down, over and over. It reads like an ignition fault to most homeowners; it's usually a sensor that needs cleaning or replacing.
Limit switches and blower faults tend to show up together, because they're related: a blower that isn't moving enough air lets the heat exchanger run hotter than it should, and the high-limit switch trips to shut the burners down before that becomes a bigger problem. Chase the actual cause — a failing blower motor, a bad capacitor, or airflow restricted somewhere else in the system — rather than treating the limit trip itself as the fault, or the same short-cycling comes right back.
Describe what the furnace is doing and we'll match you with a licensed contractor who can act on it.
Get a Furnace Contractor OutA short description is enough — we'll follow up to confirm the details and get someone scheduled.