On that model the temp gauge just pulls out. It was designed to double as a meat thermometer if I recall correctly. It is probably just stuck from many years of cooking.
Weber 22.5" Performer
Weber 22.5" One Touch Gold Kettle
Weber 18.5" One Touch Silver "Guinness" Kettle
Weber Genesis gasser
Blazin' Buttz BBQ Team - www.blazinbuttzbbq.com