Upon inspection, the furnace was found to be tripping after approximately three minutes of operation. The main limit switch, which was under warranty, was replaced as a first step. Despite this, the issue persisted. A combustion test was conducted, showing no abnormalities, and there were no signs of airflow restrictions or venting problems. Given these findings, it was recommended to remove the heat exchanger for further examination and potential replacement if needed. This thorough approach ensures that all possible causes are addressed to restore proper furnace function. Payment was collected upon completion of the work.