I have a goodman furnace modle# gupi100-4 the problem that I am having is when you turn the furnace on the induce draft motor come on the ignition comes on for a couple seconds and goes off and it keeps going on and off and never lights the furnace.I have replaced the draft motor,pressure switch and limit switch.It worked for a day and started doing the came thing

Try cleaning the flame sensor well with sand paper or steel wool.

Couldn't hurt to verify you have no blockages in your intake or exhaust. Do you have power to the gas valve to actuate?