Examined Grill unit, found far left thermostat stem mangled. Straightened out and tested. Found valve seized and knob also ruined from trying to turn on seized valve. Customer stated unit cycles off and goes into a bypass but recently stopped. GS style thermostats don't have a bypass. Spoke with Anthony. advised we will return with surface tester to make sure other thermostats are functioning properly. 1-16-19 Removed and replaced far left thermostat. Checked all 4 thermostats for calibration, all 4 thermostats within 10 degree of each other. Good to go, will part sale knob