Where is thermostat on 5.3 Chevy engine?

The 5.3L Chevy engine doesn't have a thermostat in the traditional sense of a single, easily accessible housing like some older vehicles. The thermostat is integrated into the engine's coolant passage, usually within the engine block itself. You won't find a separate thermostat housing you can readily remove. To replace it, you'll need to either drain the coolant, remove parts of the intake manifold, or possibly even remove the engine's water pump (depending on the specific year and model of your vehicle). A professional mechanic or detailed repair manual for your specific vehicle year and model is recommended for replacement.