Upon arrival, the system was running but struggling to maintain set temperature. During diagnostic testing, the furnace was found displaying a high limit fault code, indicating the unit was overheating and shutting itself down as a safety precaution. After a full inspection of airflow, electrical components, and safety controls, it was discovered that the air filter installed was excessively restrictive. The restricted filter significantly reduced return airflow, causing: • Insufficient cool air moving across the heat exchanger • Elevated internal furnace temperatures • The blower motor to run under excessive strain • The high limit switch to trip repeatedly What the High Limit Means The high limit switch is a built-in safety device designed to protect the furnace from overheating. When temperatures inside the heat exchanger exceed safe operating limits, the switch shuts off the burners while allowing the blower to continue running to cool the system down. If this happens repeat