Napolean U450 grill. Clean, with full tank, not

Check there is gas at the burners and that the orfices are not block or the burners are bot blocked .

Turn on burners let pressure out of line turn off tank remove gas line then reconnect to tank and turn on tank slowly see if this works

If your grill runs on propane, it may be a failing regulator. Try replacing the regulator and hose, not very expensive and easy to do.

Well, if turning OFF the tank valve, disconnecting the hose and wait 5 minutes before reconnecting it. And then, opening the tank valve VERY, VERY, SLOWLY (Slow motion slow) before lighting the burner farthest from the tank first and turning it to high didn't work. You need to replace the grill's hose/regulator assembly.

You can purchase one at any home center or hardware store that sells gas grills and accessories. They run around $30.00. Be sure to take your old one with you for comparison purposes.

Hope this helped you and Happy Grilling!!

Since you've done all the other things I would have recommended, it sounds like the grill hose/regulator is bad. It's all one unit, so you'll have to buy the whole thing. Lowe's and Home Depot sell generic brands that will fit your grill. Make sure you the hose/regulator with you when you go shopping, as you want to be sure to get the right length and the correct fitting size for you grill. Remember a little longer hose is alright. Shorter than yours isn't.

One final thing. When you went thru the procedure of turning the tank off and disconnecting the hose from the tank, waited 5 minutes and reconnected it. Did you turn the tank valve on VERY, VERY SLOWLY or just turn it on like you had in the past? If you just turned it on like you have in the past, that still could be the problem. You may want to try using a new tank, as the OPD Valve in the existing one may be bad. It's rare, but it can happen.

you may need a grill clean or you need to adjust the air flow to your burners

The new style OPD valves have a small brass ring located in the female fitting on the LP tank. Over time this ring can work loose and then it does not allow the male fitting to fully depress the OPD valve thus reducing your gas flow
Using a small flat bladed screw driver tighten the small brass ring This should solve your problem
IF not you might want to remove the cap (white) on your regulator and try slowly adjusting the regulator, make small adjustments and try the BBQ My Weber if I set the front and back burners to MED I get right at 350 degrees in 60 degree outdoor temps. Adjusting the regulator will take some time to get it right but If it is not your tank it is most likely your regulator out of adjustment

either your regulator is not hooked up to the tank all the way try to turn tank off make sure all knobs are turned off disconnect tank then reconnect turn tank on and try again if this doesnt work the regulator is bad

are the fire red , orange or blue?
must likely is the burner control.
check the air control at the back of the burner control, it should be open all the way.
it it is, then try another themostate.

I had this problem when I turned on the burners before I turned on the tank, to fix it I had to turn everything off. Then turn the tank on and wait about a minute for pressure to build in the lines then turn on the burners. Hope this helps

sudsy, this exact same thing would happen to me every time I changed a tank. I know it's nothing to do with the grill itself. I would have to seat and reseat the regulator hose to the tank a couple of times before it would start heating normally. Your model has 3 burners and I suspect that the flame is real low on each one even on the high setting. Try connecting and disconnecting the tank a couple of times and see if it comes to life. If worse comes to worse you can order a new regulator hose. Part number P3632L. Catriver..post back.