Heat rise is 68 degrees. Changed air filter. Found an area of concern on heat exchanger. Shut gas off and marked furnace unsafe to operate. Cleaned indoor fan. Cleaned burner system. Cleaned flame safety system. Checked all electrical commections. Leak searched coils and found no leaks. Delta T is 18 degrees. Indoor and outdoor voltage enhancement systems are within factory spec. Amperage readings are good. Cleaned outdoor coil. Indoor coil was clean. Compressor did not pass megaohm test reading 100 megaohms and is starting to fail.