Looking for ideas – IF error. Hayward W3H400FDN.    Symptoms – runs sensor checks, fan spins up, ignitor glows, gas flows, but no flame (no heat at vent and no whoosh from burner light-off).  Runs 3x then IF failure.  I took Omni-logic connections off to eliminate this as an option.

What I have done:

1>     Replaced gas valve (works, have verified flow during ignition cycle & 24V AC @ valve)

2>     Replaced hot-surface ignitor

3>     Replaced Flame sensor

4>     Checked voltages and fuses

5>     Tested flame sensor with torch

6>     Removed and cleaned burners and gas orifices

7>     Tried two Amazon  “like-new” circuit boards – both had different failure (fan didn’t run , i.e. both were worse than my existing PCB)

Any ideas?  Should the 24V at gas valve be constant or pulse?   Mine seems to supply voltage for only a few seconds – is this normal as part of the start sequence, or could I have a bad transformer (it supplies proper voltage, but I can’t tell if it does so under load).  My thought is it must be a bad circuit board, but before I spend $300+ on a new one  I wanted to see if I’m missing something else.