Rita Y;
I have a propane gas grill. A couple of years ago, I had to replace the burners. My son was visiting and actually did most of the work. However, after we tried the grill it would NOT come up to heat. I did a bit of research and learned that propane grills now have a "safety" that will shut the gas off under certain circumstances.
It is possible that your natural gas grill may have a similar safety. If so, turn all of your burners off and the main gas line. Let it set a few minutes. Then, SLOWLY, open your main gas valve (corresponding to my propane tank valve). Then, open one burner and light. Then light the others.
In my case, the safety is engaged when it thinks a hose has broken. Having all of the burners on before opening the tank sends a message that a hose is broken. The gas supply must feel some back pressure when the valve is opened to bypass the "safety".
So, all burners off. Main valve off. Open main balve SLOWLY. Light one burner, then the others one by one.
Hopefully, it is something this simple.
We have at least a couple of members here who are certified "gas people" and maybe one of them could chime in.
Let us know how things have worked out regardless of the solution.
Keep on smokin',
Dale53
