Diagnosed furnace shutting down prematurely due to overheating condition. Adjusted speed tab configuration for optimal airflow and set gas pressure to 1.5" WC on low fire and 2.9" WC on high fire. Testing revealed temperature limit switch was defective, tripping at 108°F (below 120°F threshold). Replacement limit switch (P/N 338085-402) ordered; will return to complete repair and verify proper operation.