When you say "tripping the outlet breaker", I am assuming there is ground fault rather than a short circuit.
Troubleshoot in stages:
1. Reset the GFCI on the outlet and make sure it is stable. If it still trips you might need to replace the outlet or water has gotten into the outlet or wiring.
2. If you are using an extension cord, unplug the grill from the extension cord and then plug the extension cord into the outlet. Make sure it is stable. If it trips, replace the extension cord with one rated for outdoor use.
3. Now try plugging the grill power cord into either the extension cord or directly into the outlet with the opposite end being disconnected from the grill itself.
I presume you will still be good at this point.
4. Plug the power cord securely into the bottom of the pellet box. This cord can easily come loose and might cause a GFCI fault if not plugged in securely. Make sure the on-off rocker switch is in the off position. If all is well with the cable plugged in then turn the rocker switch on.
I have read reports of the on-off rocker switch on the grills going bad. If you have soldering skills, the switch is easily replaced. If not, you can purchase a switch assembly from Traeger.
5. Now try to power up the controller. Does the GFCI trip immediately? If so, it is going to take some more complex troubleshooting. You will need to access the controller panel and unplug the wires going to the fan, auger motor, and hot rod. Before unplugging any wires, be sure the power cable is unplugged from the unit and make a note of which color wire is connected to each part of the grill so they can be reconnected properly. Leave the black and white cable connected as it supplies power to the controller.
With these wires unplugged, plug the power cord into the grill as see what happens, If the GFCI still trips when you turn it on, then the problem is either with the controller itself of with one of the sensors such as the RTD temperature sensor. If the controller will turn on successfully, one by one plug the other cables back into the controller. Be sure that the power cord is unplugged before reconnecting each wire. If the GFCI trips, then the last wire connected is the one going to the problematic part. It will need to be replaced,
Since your grill is 10 years old, it is likely that insulation has broken down on one of the parts allowing current leakage to ground. This will cause the GFCI to trip. While it can be aggravating, it is for your own safety.
Now try to turn the controller on, but do ignite the grill. Does the GFCI breaker trip as soon as the
Do you have a digital voltmeter that measures resistance?
If so, you can measure the DC resistance between each o