I was working on a bradford white hot water tank today the M# is MITW40L6BN15

Tank would ignite and run for 10 to 20 sec and flame would shut off, it would do this over and over without locking out, I replaced the flame sensor/ignitor assembly, still does same problem, pressure switch and vent limit switch both stay closed when flame goes off so it is neither of those, i even ran it without the vent and it did the same thing, i ordered the smart valve that goes on it today because thats what i think the problem was
smartvalve # is SV9501M5109
looks like the same one on the older armstrong furnaces (brown smartvalve without the led light) but its slightly different for the hwt
any help would be appreciated thanks