Service Engine Soon light came on and Temp. Gauge dropped to 100deg. Car still running fine, heater still works good. Could not find anything physically wrong with the car. What would make the light come on and the gauge drop?

Service Engine Soon light came on and Temp. Gauge dropped to 100deg. Car still running fine, heater still works good. Could not find anything physically wrong with the car. What would make the light come on and the gauge drop?
Lets start with the coolant sensor. Happened to mine. Same siganals your getting but in my case the coolant was low causing the sensor not to read correctly. Then after the mechanic was looking at it found a leak.The intake manifold was cracked right were the sensor screws in causing DexCool toleak out. 1 big job and 1 new intake later the car is better. Tons of problems with these manifolds and gaskets. Hop on it winters coming !!!!!!

Lets start with the coolant sensor. Happened to mine. Same siganals your getting but in my case the coolant was low causing the sensor not to read correctly. Then after the mechanic was looking at it found a leak.The intake manifold was cracked right were the sensor screws in causing DexCool toleak out. 1 big job and 1 new intake later the car is better. Tons of problems with these manifolds and gaskets. Hop on it winters coming !!!!!!
Thanks for the info!!! the sensor was my first thought also. But, i'm not all that up on these newer autos. I could not see any visiable signs of leaking but will give it a hard look, Thanks again...

I just recently had work done (either october or november this year) on my 01 to get the crack in the intake manifold fixed... and just today I noticed while driving home, the temperature gauge was stuck at below 100 and didn't move at all.. the service engine soon light came on shortly after... I checked the coolant levels and nothing was leaking out.. Any ideas... seems to be mine did the reverse of everyone elses... I also noticed when I turned the car off, it sounded like I was landing a space ship, it had this whurring noise.. I would assume had to be a fan of some sort...

Welcome to AF newbies !

Hummer1959 and cass61182, did you get the car scanned?
Best bet is to , and then you'll know a little more into whats going wrong.


Heres a pic of the coolant temp sensor, and it located right under the TB on the V6 easy to see and get to.
V6 temp sensor (http://i28.photobucket.com/albums/c242/xeroinfinity/11X17RW24/tempsensor.jpg)

4 Cyl Temp sensor (http://i28.photobucket.com/albums/c242/xeroinfinity/11X17RW24/coolantsen4.jpg)

Testing your Coolant temp sensor is as follows;

- First inspect the wires at the plug for looseness or breaks.

- Now test the circuit for power from the PCM. Disconnect the plug-in on the sensor and test the two terminals on the wire harness.
Connect the + lead on the voltmeter to #1 and the ground to #2. Turn the key on, should read approx 5 volts. If not check the wiring from the plug to the pcm.
If the cicuits are good have the PCM checked by a stealership.
http://i28.photobucket.com/albums/c242/xeroinfinity/11X17RW24/coolantsenplug.jpg

- With the ignition off and engine cold, unplug the sensor connector.
Using an Ohmmeter measure the resistance between the terminals on the sensor cold(between 50-80 deg). Write that # down.

- Start the engine and let it get to running temp between 180-200deg.
Disconnect the connector and test the resistance again.
Compare the two measurments to the chart below.
http://i28.photobucket.com/albums/c242/xeroinfinity/11X17RW24/tempresistancevalues.jpg
- If the results are incorrect then replace the sensor.

When replacing the sensor use teflon tape to seal from leaks.

Hope that helps.