The plumber and I have both talked to tech support and they have the same answer - replace part A, B, and C. Those parts have already been replaced and I doubted the originals were faulty to begin with but when you start running out of ideas...
But to let you know specifically what we're dealing it's a Cleveland CSM24 Steam Coil Convection Oven. It uses building steam that runs through a coil in a boiler beneath the two heating compartments to heat the food grade water in the tank into steam. The building steam itself is not food safe which is why this system is used.
The basic problem is that the appliance operates for 3-4 hours and then shuts down. During the period of time when it fails we see the water in the sight glass go down but it doesn't appear that the probe in the tank that maintains the water level is signalling the circuit board to open the water fill valve. It takes approximately 20 minutes for the water to almost disappear from the sight glass and then the steam shuts off and it goes into a MANUAL reset mode. After it does it the first time it tends to happen every hour thereafter.
I placed 3 DMM's on the high probe, low probe, and water fill valve to see what was happening as far as if the signals were being sent. Every time we watched it shutdown we observed no change in voltage which despite the water level in the sight glass the water in the tank appeared to be covering the 2 probes and the water fill valve remained closed. We kept working on it from the point of that probes or circuit board were faulty but only during this time for reasons we could not explain.
The other day I came up with an idea that there was a counter pressure forcing the water in the tube to go down while the water in the tank remained high and eventually this mystical pressure caused it trip out on high limit. I metered the high limit during failure only to be proven wrong. But despite being told that my basic idea was scientifically impossible I found the smoking gun. Last night after posting this topic I finally found the supporting science to prove that water level can be high in the boiler while the sight glass level is lower and this eventually leads to a shutdown!!!https://www.youtube.com/watch?v=a0xZPl_bwHI&feature=youtu.behttps://www.youtube.com/watch?v=tI3tWuSsX7c