Diagnosis
Temperature runaway. The controller is not modulating the fan correctly, often due to a dirty or faulty temperature probe.
How to Fix It
Clean the RTD (temperature) probe in the cook chamber. Let the grill cool. Remove the probe from the left side wall. Scrub it with a brass brush or fine steel wool to remove grease and carbon buildup. Reinstall it snugly.
If That Didn't Work
The damper on the exhaust chimney may be stuck closed. Manually open and close it a few times to ensure it moves freely. It should modulate to control heat.
Heads Up
Always start with a small, controlled fire. If you light too much charcoal initially, the grill will overshoot its target before the controller can catch up. Use the recommended startup amount.