Performed heating tuneup. During inspection, found ductwork very dirty. Hot surface igniter has worn marker visibly showing, this is indication HSI is close to cracking. Combustion motor is running within proper amp rating but bearings are grinding very loud, this is a sign motor is starting to fail. Failed powder cloud test- Combustion chamber should be sealed and not allow any airflow through heat exchanger. Inspected HX and did not visibly see any cracks, however rust is rampid on clamshell. carbon monoxide test is running at 4ppm of CO during inspection and system tripped high limit. Verified airflow was correct. Findings are indication of failed HX- overheating main limit. Recommend replacement of furnace due to severity of issues found and cost of repair vs replacement. System is 10 years old.