Diagnosis
The controller's P.I.D. settings are off or the RTD probe is dirty. Grease and carbon insulate the probe, giving false readings.
How to Fix It
Clean the RTD probe thoroughly. Let the grill cool. The probe is on the left interior wall. Scrub it with a brass brush or fine steel wool until the metal is shiny. Do not bend it. Also, vacuum out the fire pot and the entire bottom of the cook chamber. Excess ash causes erratic burns.
If That Didn't Work
You may need to reset the controller's calibration. For many Z Grills models, you hold the 'Menu' and 'Prime' buttons together for 3 seconds. Consult your specific manual for the 700D3 sequence. This resets the learned temperature parameters.
Heads Up
Always let the grill preheat for 15-20 minutes with the lid closed before trusting the temperature reading. Lid-open checks ruin the cycle.