The status LED shows 4 flashes; the furnace starts but shuts down shortly after ignition, or it will not ignite.

Safety: Turn off power before removing the blower door or working near wiring.

Steps to check:

• Replace or clean air filter: Locate the filter in the return air grille or furnace filter rack. Replace it with the correct size and airflow rating. A severely clogged filter is the most common cause of high limit switch trips.
• Check airflow: Ensure all supply registers and return grilles are open and not blocked by furniture, rugs, or drapes.
• Inspect blower operation: With the thermostat calling for heat, verify the indoor blower starts promptly and runs at normal speed. Listen for unusual noises or slow ramp-up. If the blower does not run, check the blower motor, capacitor, and the control board's fan relay.
• Inspect high limit switch: Locate the main limit switch on the heat exchanger panel. Check for loose wiring connections. If the switch repeatedly opens even with good airflow, it may be weak and require replacement with a Ruud-approved limit switch.
• Check filter rack and ductwork: Look for collapsed flex duct, closed dampers, or undersized return air ducts that could restrict airflow.