Assuming the valve in question is the one that supplies gas to the grill’s burners and not the gas regulator on the propane tank or other fuel source, valve replacement is easy. Check the owner’s manual and buy the valve specifically designed for your make and model of grill.
If the gas valve on the grill really has gone bad, then replace the whole assembly. Sometimes this may mean replacing the control panel as well. Many gas grill manufacturers provide step-by-step instructions for repairing grills in their manuals and online websites. Follow the maker’s instructions, and many valves simply screw in.
Before buying a replacement valve for your gas grill, however, inspect the current valve. Check to see if the valve is working correctly, the valve knobs should turn and release completely. They should depress fully and pop up freely. Lubricate sticking knobs with a solvent such as WD-40, and work them to distribute the lubrication. Truly worn valves tend to bind and must be replaced to prevent flashback fires.
Next, check the hose connection and valve orifice. Tighten the connector if necessary, and then check the valve orifice for obstructions such as spider webs. In a similar manner to soot build-up, clogs can cause poor gas flow, poor heating and other performance issues in the valve, as well as the burner tubes connected to it.