Responded to callback service request for thermostat displaying error code 53. Inspected and secured all wiring connections at thermostat and control board, verified proper voltage readings throughout system. Performed comprehensive diagnostic testing including heat exchanger inspection, which revealed rust buildup on one heat exchanger eyelid. Carbon monoxide levels and gas pressure (both low and high) tested within acceptable safety parameters. Informed homeowner about heat exchanger condition and explained that thermostat lacks reset functionality for the specific error code.
