I have a Weber Genesis Gold C gas grill that I bought new about ten years ago, and have never had a problem with. Recently, it would not light with the sparker (even thought the spark was visible and strong), but would light with a handheld butane lighter. The flame on the front bar was however too yellow and flickering. The other two bars were OK.
Now, this situation is covered in the manual, a common problem being a spider web in the venturi. I took all three burner bars out, cleaned their plastic bug screens, and looked down the bore but saw nothing, and put it all back together.
Now, all the bars have low flame, flickering and noisy. Huh?
Sounds like the regulator's max-flow limiter tripped, don't know how or why. Went through the reset sequence; no change. Did it again, no joy.
Bought a replacement regulator for $25. It would not turn on at all, don't know why.
Went through the sequence again with the original regulator, this time rapping on the regulator body with a lead hammer. This worked.
But still the front bar is yielding a too-yellow flame, and the spark lighter does not work. Used the grill this way a few times - the grill ran cooler than normal, but I was able to compensate.
When the weather cooperated, swapped the front and rear burner bars (they are identical, but one is upside down with respect to the other). Now, the yellow flame is in the back, so the problem is in the burner bar, not the burner gas valve.
Now, burner bars are dead simple, being a bit of pressed stainless steel sheet. Took the bar into my shop, took the bug screen and air adjustment gate off (first marking its location using a metal scriber), and looked down the bore with a flashlight. Sure enough, there was a spiderweb in the venturi throat. The web was thin enough to see through, and yet dense enough to interfere with correct mixing of air and fuel. Cleaned the venturi out with a big screwdriver, and put everything back together. Now the grill works normally.
And the moral is that one should clean the burner bar venturi throats even if they don't seem to need it.
Joe Gwinn