DIAGNOSIS The controller is throwing a high-temperature error. The RTD probe in the cook chamber is likely reading incorrectly or is disconnected.
FIX Unplug the grill. Open the main hopper lid and remove all pellets for safety. Remove the cooking grates and heat baffle to access the rear of the cook chamber. Find the RTD probe—it's a thin metal rod sticking into the chamber from the left side wall. Check its connection. The wire runs to the back of the controller. Ensure that plug is fully seated. Plug the grill back in. If 'err' is gone, reassemble.
IF THAT DIDN'T WORK The RTD probe itself may be faulty. You'll need to test it for continuity with a multimeter or replace it. Part number is usually ZPG-7003-0011 or similar for the 700 series.
HEADS UP Never bypass or run the grill with an 'err' code. It can cause a dangerous over-fire situation in the fire pot.