My 2004 Silverado 1500 Base cab heat fluctuates. No indication of overheating, gauge stays about 190°. Is this a defective Thermostat?

I used Google as to the functions and purpose of a "thermostat" which I must assume, since you did not explain, is the device in the engine that serves to block cold coolant into an engine so that the engine reaches normal operating temperatures faster. A thermostat is a simple device that reacts to temperatures. This function has nothing to do with the "cab heat" that you mention. The invention to perfection of the cabin temperature devices in automobiles is rather long. The heat function is the most simple one, if it can be called that. Coolant is diverted into a what must be names a radiator inside the cab. Controls set by the person inside the car determine the temperature possible. There is a fan controlled from OFF to the engineered settings, usually, 1 through 4. Other controls can divert the airflow to the various vents from those directed to the front windscreen to passengers to the feet area, with some models having percentages possible as to two or more of the vents. Vehicles, for the most part, have engine temperature gauges or have an On Board Diagnostics (OBD) ports, either I or II, that an auto technician can hook a device to read the temperature of the coolant. Since you mention the temperature of this vehicle stays at the reported 190 degrees, then the operation of the thermostat cannot be questioned. I looks as if you did not read up online as to the function of the thermostat as I did. May I ask why? The age of the vehicle and the probably lack of service to the coolant system, I suspect this is a "new" vehicle and as to why a 16 year old kid requires a truck to go to high school classes is a different posting, may be a factor. The tiny radiator type can get rust and deteriorate. But there are other controls that can affect the available heat. As to why you elected to text here versus contacting a local auto technician, that is explained by the smartphone/texting addiction that prevents kids from using brain power over smartphone power to think of the issue and to proceed to the most logical place for resolution. That is a qualified mechanic. Many do some seriously flawed logic. They change thermostats, they flush the system, some even change spark plugs and wires because that is what other children with cars tell them. The avoidance of mechanics is rather contagious and serious. No, the thermostat is not the problem. No one can tell the problem without an inspection and evaluation by an auto technician. May I suggest this Silverado (thanks for capitalizing car names correctly) be driven to an auto technician for repairs? Oh, the term "thermostat" is not a title but a generic term. As such it cannot be capitalized. Good luck.

If by fluctuates you mean the heat blowing out of the vents gets hot when you are driving but gets cold t idle speed you have an air pocket in the heater core or the DexCool is plugging the heater core. I'd pull the two hoses off the heater core at the firewall and flush the heater core with a garden hose in both directions. Just clamp off the hoses before removing them so you don't have to drain the complete cooling system. Make sure to run the motor for a good 15 minutes after flushing the heater core. Have the climate control set on the hottest setting and have the reservoir pressure cap off until all the air is purged.

It is working properly the operating temperature is a190 degree thermostat, if you like you can look up a replacement match at any parts store web site. Also just for general knowledge all vehicles produced for many years all have "Fail Safety Thermostats" which means when they go "Bad" they automatic go to the "Open Position" to stop engine overheating due too "Thermostat Failure"......these type have been around for +20 years!

I would say no on thermostat since your running 190 degrees. Either temp door issue, heater core issue or low on coolant

it will do that when the themostat opens it releases the cold water fom the hot to cool the engine and that will throw the temp off a bit if it stays warm your good