One of our guys performed pm on a two system house, got a call a day later that the second floor system wasn't working/keeping up. He worked on it for a while, I was told to go see if I could figure it out. Here are the details:

Call for heat, inducer kicks on, pulls 1.97" wc, ps needs 1.35 to close. Ps closes, ignitor heats up. Gv energized, opens for less than a second and closes. Not open long enough for the flame to flow acrossed all four burners, literally less than a second and the gv is de-energized by the board.

Call for heat does not drop out.
Pressure switch does not drop out.
Inducer does not drop out, continues to pull 1.87 to 2" wc throughout the cycle.
No limits are opening to cause interruption.

Burners are clear all the way acrossed, flame sensor is clean (not on long enough for flame sensor to come into play at this point)

Here is the kicker, twice while trouble shooting the furnace lit and stayed lit. Once I shut it down and attempted to refire, problem reoccurred.

Have a new board installed at this time, same issue.

Furnace is a 94 Trane 90% upflow.

Any ideas/thoughts are greatly appreciated!