This unit uses flame rectification, this is a DC signal of micro amps that returns to the ignition module via grnd/sense terminal. If it has no flame sensor, it uses the burner /pilot assembly's bonding which transmits the weak DC signal to the module to prove a flame exists and is safe to continue operating. I have a routine I use for these , I clean the pilot assembly's mounting brackets surface areas and the burners surface areas touching the pilot assembly mounts, basically we are making a nice conductive path to home , being the grnd/sense terminal, some say burner ground. In adition I use 105C degree rated wire and connectors and run a ground wire in addition to the units grounding back to the terminal that is used for sense or grnd, usually they are coon in the boards. Remember it is only transmitting the faint signal that passes through the flame, a flame is a poor conductor and only transmits microamps DC and that is easily stopped by a patina or moisture or corrosion etc, it does not take much and it is easily disrupted and restored. In fact many times new components seemingly rectify an issue, often the restoration of a ground path for the flame signal is really what fixed everything and if you try the old equipment it is operative about 40% of the time according to manufacturers. Johnson controls makes a nifty flame simulator that simulates a nice DC signal that will work on any flame rectification system! it plugs in with the existing flame proving circuit and clamps to ground with a clip, If it wont run with this, no question the boards/module is defective. For the pressure switches I use tattle tales to tell me if any control opens,even if a brief second, that is hard to spot unless gazing at it while it pops opens for 1 second , pressure switches on the threshold will waver and not be evident by watching , tattle tales light up if opened.
If this continues after typical protocols a in depth analysis must be made noting the water column measurements of the intake vs exhaust which the pressure switches monitor for correct differentials. The flame sensing issue is much less complex.The 2 pressure switches monitors components that must operate within parameters deciphered by a differential between the 2 being maintained.
So the flues routing, sizing, length and termination must be analyzed, as will the combustion air makeup to the mechanical room/home/area unit is located and while under worst case scenario condition, meaning all bath vents, kitchen vents and dryer and Jenn aire etc etc must be on and doors closed, and see if a negative pressure condition ever exists and why.
There is a systematic analysis protocol to be done still, the techs there just have never been shown how these things actually run.