I have replaced the upper plenum, fan, pressure switch, ignitor, flame sensor, and I cleaned all 8 burners of dirt dobbers and rust. The heater worked a few times and now one month later I again get error code 004 after it tries to cycle on 4-5 times. After the purge cycle, the fan shuts off for about 15 seconds and then I hear the fuel gas ignite. When the fan does not turn back on shortly after ignition, the fuel gas trips off. After 4-5 times, I get the E004 code. Can someone tell me what to do to get this problem fixed. I do not see anything wrong with the hose. Also, I cleaned the burner valve as best I could.