Troubleshot and repaired an American Standard 80% furnace with intermittent operation. Diagnosed failed flame rectification circuit causing random shutdowns. Multimeter showed microamp reading of only 0.8μA (should be minimum 1.5μA). Replaced flame sensor, cleaned burner assembly, and adjusted gas manifold pressure from 3.2" W.C. to factory spec of 3.5" W.C. Also found partially restricted condensate drain line causing pressure switch trips during heavy operation. Cleared drain, installed clean-out access point for future maintenance, and added condensate inhibitor tablets. Full system operation verified through complete heating cycles. Unit now operating at specified parameters.