If you have a newer grill or burner, or your burner appears to be fine then the problem might be with the regulator. The regulator is a round metal object near the connector to your gas supply. Sometimes regulators get stuck. This can reduce the gas flow, make a yellow flame and keep your grill from getting hot. To solve a stuck regulator, turn off the fuel supply to your grill. Disconnect the main connector and open the grills control value. This will eliminate any pressure on the regulator. Sometimes this will reset the regulator, but sometimes they stay stuck. Take a small hard object like the handle of a screwdriver and give the regulator a gentle tap.
Now you can reconnect everything and see how your grill works now. If you still have a yellow flame your propane tank might be low, the regulator may need replacing or you could have a blockage in the venturi tubes. So check your fuel level by pouring hot, not boiling water down the side of the tank. Feel along the side where you poured the water. The propane level is where the tank feels cold. If you have plenty of fuel, locate the venturi tubes. The venturi tubes are located along fuel lines right behind the control valves. These allow air to mix with the fuel before reaching the burners so they have a small open space. If that space is blocked it will restrict the fuel flow to the burner. Make sure it is clear of obstruction.
If you have done all this and still have a yellow flame you may need to look at replacing your regulator.
