So my wife's hummer is running a little under 3/4 mark. Took to a mechanic shop he tried radiator fan clutch and thermostat. I got it back and told him to put stock stuff back on because it didn't help. It had a cel which was coolant temp to low so I replaced coolant sensor. Its still running warm. What else is there to check. Thanks in advance

 

Everyone's H3 reads a little different on that guage. Anywhere between 1/2 to 3/4 is normal.
Those gauges are not precision instruments. I wouldn't be concerned until the needle is in the RED.

If you really need to know if you are running over temp, then plug into the OBDII port with a scanner or reader to get the actual temperature while running. (something the mechanic should have done before just swapping in new parts)

 

I recently posted some fine detail on the gage and obd2 temps. the gage is pretty linear from a bottom of 90 f to a mid of 195. about 100 degrees for the low 1/2 of the gage area, pretty good accuracy and I can only assume it would continue above 195 on up to ?290 at the top! (another 100!)

I cant make mine go all the way to the mid point, just a notch below is 195.

I read 201 only when I turned the engine off and let it heat soak. I even had a blanket blocking the grill for a while. never 200 when running,never mid point.

 

He said the temp was at 229. I recently changed radiator 3 years ago do to leaking so I knew that couldn't be it. My thinking is with the code reading to low, the car was over fuelling making it hot. Guess I need to re check actual temp again. Do these cats clog up with out throwing o2 sensor codes?