stormy down

Red "Thermometer" warning light came on yesterday while driving my '96 V6 A6.
Topped up coolant (to MAX) when cool at home and let car heat up at idle. Water and Oil Temp gauges came to normal readings in usual amount of time and stayed normal even when the Warning Light came back on and the coolant was on verge of "blowing" out of expansion tank.
Radiator fans are operational although not kicking in. I by-passed the fan relay and let them run but radiator did not appear to be hot so I don't they're the issue.
With a hot top radiator hose and a barely warm bottom hose I'm thinking it's the Thermostat stuck shut. The only problem with that thought is that on other cars I've owned, when the Thermostat was stuck, the Temp gauge continued to rise all the way to HOT.

If it's not the Thermostat, what is it?
Any ideas?? Appreciate any help.

Tony

97A612V

coolant expands when it gets hot.. when the car is cool the coolant level should be in between the min and max line, when hot it should be slightly over or on the max line. im pretty sure the light will go on if there is to much.. i jsut replaced the radiator on my 97 a6 becuase it cracked, but i have the sensors. if they end up being your problem contact me at mrrobottoe@aol.com and we can help you out with them