I have a goodman funace model number gmp100-4. The problem I am having is it turns on then turns right back off. I removed the inspection cover to look at the pilot lighting switch which seems to be ok but seen some corrosion on the gas line switch pretty sure that this is the problem but wanted to know if this is something that a professional should change for me? the corrosion looks like it came from the vent leaking back into the case,this switch sit directly under it. what can I do about this also

Vacuum the area if possible.

Sounds like the debris or dirt has blocks the signal from the flame sensor. Clean sensor with sand paper or replace.

NOTE: I never recommend a home owner dissemble a furnace including removal of the burners to clean. That should be left to a professional since the burners have to be placed exactly as removed. Goodman also had some burner problems and after a time they will need to be replaced.