I have a new Laars Mascot combination boiler/water heater. It fires for a second, then shuts down. After 3 attempts, it shuts down on error code 133.

A boiler tech looked at it, seemed to think the flame sensing electrode was working fine. They decided it was the control board, but they just put a new one in and it didn't fix anything.

Like I said, it gets spark and fires fine. After a second, the gas valve shuts off.

Help! I've been without hot water for over 3 weeks at home now.

I'm not familiar with the Laars, however something similar has happened to me on a recent install.
Have tech check gas pressure with a manometer. Try to start boiler if pressure drops significantly it may be the gas piping/equipment. If propane , new regulator by the tank needed.