Where is the engine coolant temp sensor located on a 2004 grand prix gt

Lucas, Watch the video link and it will show location of sensor.
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
How Replace Engine Coolant Temp Sensor 2003 Grand Prix
GM 3100 3400 3500 Engine Coolant Temp Sensor Replacement
2003 grand am temp sending unit Google Search

Check Sensor in Reservoir also the wiring to it.

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 .

Yes that is what the code means. GM usually uses two, one that makes the gauge work and one that tells the ECM what the engine temp. is. You need the one for the computer.

its located at the rear of engine on driverside, heres a picture that lists a few sensor locations, d = the coolant temp sensor, good luck.

the ect (engine coolant temp sensor) is located at the top left rear of engine, below throttle body.hope this is helpful.

It looks like this with a connector to it and is on the side of the engine block:


Look right under the radiator hose.

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.

possible IAC dirty, throttle plate dirty, Coolant temp sensor stuck and flooding conditions occur.

Replace the temp sensor/switch located on the radiator.  Don't forget to drain your radiator and use teflon tape on the sensor when installing it.  DO NOT over-tighten.