I noticed my engine light on the dash of my 2004 Grand Prix was on so checked code with tester. Said coolant temp below thermostat temp. Replaced thermostat drove car for 30 minutes. Popped hood fixed top hose leak with clamp. Then noticed the gauge in car registered nothing as in cold and the two fans run all the time. Could this be that I need a new engine coolant temp sensor?

Don't think so... I had one 2004 prix that lost it's coolant and it ran until it destroyed itself... Had another that would hit red on temp during the summer in the desert mountains. Never shut down on me though.... replaced the radiator, water pump and thermostat. My fans kick on about 3/4 of the temp gauge. Normal temp is between 1/2 & 3/4.

Replace your coolant temperature switch,it's screwed into block by oil filter.Here's a picture of what it look's like.Your water pump working properly.Are your spark plugs burning clean,not oily from bad piston rings?

Yes , having the vehicle checked for DTC'S - diagnostic trouble code's should always be the first step in diagnostic process when any warning lamps are lit on the instrument cluster . Would be best to have someone with a professional type scanner , that can view engine sensor data . The engine coolant temp. sensor is a input to the PCM - engine computer . Can see what the actual temp. of the engine is . GM had bad stepper motors inside the instrument cluster's 2003 to 2006 , these stepper motors control the gauge needles . Testing what the real temp is an not going by temp gauge . harbor freight tools sells a tool under $30 bucks that can check engine temp . Infrared Laser Thermometer .. Point it at the thermostat housing . Can use it to tell what temp. the thermostat open . But , testing is the key ! Not guessing .

On the 3.8 liter the coolant temp sensor is right below the the thermostat in the intake manifold on the drivers side of the engine.

The thermostat is under the housing that is bolted to the intake plenum that the upper hose is attached to.

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CHECK COOLANT LEVEL.IF COOLING FAN TURNING ON COOLANT TEMPERATURE SENSOR WORKING.BECAUSE IT CONTROLS AND TURN ON COOLANT FAN AT A SET TEMPERATURE.SOUND LIKE ENGINE RUNNING TOO HOT. HAVE RADIATOR FLUSHED.REPLACE THERMOSTAT. HAVE ENGINE BLOCK FLUSHED.REPLACE RADIATOR PRESSURE CAP ALSO.SEE WHAT HAPPENS.

Could be the thermostat. I know nothing about cars but I noticed my 2000 Grand Prix temp gauge was not going to the normal level. My wife drives this car so I didn't notice it for a month or so. Eventually the Engine light came on. Took it to Autozone and the code came back as P0180... (1) low coolant (2) bad thermostat (3) enginge fuel temp sensor. They used a code reader that was about the size of an Ipod nano.

Coolant was okay, so took it to my mechanic. His netbook sized computer showed it was the Thermostat. He reset the codes and the temp guage went back to normal. Was short on $40 for the fix at the time, so I parked the car and promised to bring it back in 3 days. Went out and checked the car a day later at 25 degrees outside without starting the car... gauge still sitting at the normal level. No question it's the thermostat. That car will not be driven again until T.stat is fixed in 2 days.

What your describing is either a sticking thermostat, a low coolant condition or a blown cylinder head gasket.The code is for coolant temp fault.Not always the temp sensor but it is located next to the thermostat housing.

the temperature sending unit on a 97 grand prix 3800 gt(which is also the coolant temperature sensor) is not accurate. the sending unit part of this sensor only sends info to the guage on the dash. the coolant sensor part sends info to the computer of the car. it is located under thermostat and somewhat difficult to get at. i had put a brand new one in and my temperature on my dash is all over the place---even with good thermostat and properly bled system. when i hook up a scanner to my computer, the coolant sensor is sending the computer(ecu??) what seems to be the right temp. anywhere from 194 degrees to 217 when hot. the guage however on dash go from 160 to 260 in less than 2 minutes.

more than likely the thermostat has stuck open. Or opening to soon not allowing the car to reach normal operating temperature. I would start by replacing the thermostat. Clearing the code and running a drive cycle and see if the code comes back. if the code does not return problem solved. if it does return replace the coolant temp sensor.

check evap vent solenoid alot stick on these cars.