GM-GMC: Engine does not heat up, engine coolant temperature, radiator hose

Question
The engine on my 99 Sierra 4.8 will not heat up over 170 degrees on the gauge anymore, even after 45 minutes of idling.  Does not heat up from normal driving either. Very little heat coming into cabin.  Replaced thermostat with no improvement.  Changed thermostat the 3rd time with one from the dealer and still the same.  I do not think it is the gauge because you can hold onto the radiator hose and feel that it is just not that hot.  Also flushed heater core just so that I could get the most heat into cabin as I can from what little is there.  My mechanic and I are clueless at this point.

Answer
The gauge can be deceiving.  I would recommend putting a scan tool on it and reading what the actual engine coolant temperature is getting to.  There is nothing else that controls the engine's operating temperature.  Is it possible that you have a HVAC problem, if the air is not routed over the heater core, you will never get warm air in the cab.
Just a couple thoughts. Hope it helps