*** Edit - was missing the second photo... operator error***
Flame is sensed from ignitor through flame to burner and/or screen ground. If ignition module and ignitor are both new, the only variables that should effect flame sensing are how well the burner assembly/screen is grounded and whether the flame is hitting both the ignitor and the burner assembly/screen correctly to sense the flame. Check burner/screen ground. Check gas orifice. I don't know if the burner and the burner screen are one piece or separate, but it seems if the ignitor is outside of the burner/screen assembly that the flame is actually sensed between the ignitor and the burner screen because the gas/air mixture between the burner and the burner screen would not be lit. If possible check that the outside of the burner screen is clean bare metal and grounded well. Check the inside of the burner for dirt/blockage. If possible check the burner slots/orifices for blockage/cleanliness. I have worked on both residential and commercial units with drum/cylindrical combustion chambers and negative pressure gas valves and they seem to have some specific areas with problems. Most notably corrosion/dirt in the combustion chamber and also around fasteners for various parts attached to the combustion chamber.
Swamp,
Lot of good stuff to check here. Thanks so much. I have removed the burner tube and it was in pretty good shape but I sanded it clean and shiny anyway. The screen is as you suspect. Here's a photo I took (before cleaning):
Here's the bottom that is close to the bottom of the firebox (also before cleaning):
I also measured the voltage at the test posts on the Fenwel controller. Never gets close to the 2 volts or so. I measured a max of 150 mv after the flame lights, then it drops down to a couple mv before the gas valve closes. I'm really wondering if maybe the fan is running too slow. Would definitely make the flame low. My next test is to parallel the flame sense outside the fire box. I'm thinking to tie on the the black wire on the igniter with a nail at the end of a board and then to run a ground wire from by blow torch to the ground on the ignition control board. Then when the gas valve opens put the nail in the flame. If this makes the blower stay on then maybe my flame is actually too small and more evidence I should buy the new blower. Also, I definitely know the connector going back to the blower from the ignition module (terminals F1 and F2) got totally fried.
Any thoughts on this approach?
Chris