DIAGNOSIS Insufficient airflow or a dirty temperature sensor. The grill regulates temperature by modulating the fan and pellet feed. A blockage limits max heat.
FIX Perform a thorough clean. Scrape the fire pot completely empty of ash. Vacuum out the ash from the bottom of the cook chamber. Use a grill brush on the temperature probe (a metal pin located on the left side wall). Ensure the grease tray and drip pan are not overflowing, as this can choke airflow.
IF THAT DIDN'T WORK The thermal fuse might be tripping prematurely. Located near the fan housing, it can fail if the grill overheated once. A failed thermal fuse must be replaced to restore full temperature range.
HEADS UP This grill is designed to max out around 500°F for grilling, not like an oven. Consistently hitting only 400°F indicates a problem, but expecting 600°F+ is unrealistic for this model.