The status LED shows 11 flashes; the furnace may lock out after several attempts, and the blower may run.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Chronic airflow restriction, an oversized furnace causing short cycling, a weak limit switch, or blower speed set too low for heating.

Safety: Turn off power before accessing the blower compartment or wiring.

Steps to check:

Confirm filter and registers: Ensure a clean filter is installed and all registers and returns are open.
Check blower speed tap: On PSC blower motors, verify the correct speed tap is connected to HEAT on the control board per Ruud's airflow chart. A too-low speed can cause overheating.
Inspect ductwork: Look for crushed or disconnected ducts, closed dampers, or zoning issues that restrict airflow.
Evaluate furnace sizing: If the furnace frequently cycles on the limit switch even with good airflow, it may be oversized. This requires evaluation by an HVAC professional.

Hard 4/5

High Limit Switch

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