Today to perform one system HVAC service call. Upon arrival, customer stated that their system would turn on and then cut back off in heat mode. Pond inspection of the unit found that the flame sensor had buildup on it, causing it not to sense the flame, the electrostatic filter was completely impacted with dust and debris causing the system to shut off on high limi. After cleaning the flame sensor and the filter and starting the unit back up, everything ran properly. Upon further inspection found the blower wheel to be dirty, the flue pipe was not up to code, and there was no flood protection for the system. Also found rust spots in the heat exchanger. Upon completion of a combustion analysis found that the heat exchanger is still operating properly. All other heating elements were working properly. Upon speaking with customer made customer aware that the heat exchanger does have rust spots, and that this could lead to a failed heat exchanger. Customer declined any repairs at this tim