I have an 04 3.0 v6. Temp gauge started going to the cold side and the check engine light came on. A mechanic friend said the temp send unit is bad. Replaced it and still same problem. engine runs perfect just cant tell the temp. where is the unit located. i found 1 towards the right and rear of thermostat housing and another directly in front of the thermostat housing. any help? thanks

 

The temp SENDER for the dash board gauge has 1 wire, it uses the threads as the Ground for the circuit, never wrap lower threads with "tape", you will end up with intermittent or bad gauge readings.

The temp SENSOR for the computer(PCM) has 2 wires, it is only used by the PCM to set "choke mode"(Open Loop) and then "Closed Loop mode".

There is no connection or cross over between these two separate devices, 1 wire SENDER, 2 wire SENSOR.

Choke mode is when PCM runs engine rich with advanced spark timing and high idle(1,000rpm approx.)
Closed loop is when the PCM starts to use the O2 sensors to set leaner mix for best MPG, idle about 750rpm.

The PCM has a timer with the ECT sensor circuit, after startup if the ECT sensor does not report 190degF engine temp after 5 minutes the CEL(check engine light) will come on.

This can happen for a few reasons, yes, ECT sensor could fail, but sensors rarely fail, just FYI.
What is more common is that the thermostat has failed, these are suppose to fail OPEN, so engine doesn't over heat.
If thermostat has failed then engine can not warm up, which is what your dash board gauge shows and ECT sensor timer reported.

So my guess, and it is just a guess from what you have posted, is that your thermostat is stuck open.

To test if this is true, start cold engine, feel upper radiator hose, it will be cold.
Let engine idle for 3 or 4 minutes and feel upper hose again, should still be cold, if it is warming up with the engine then thermostat is stuck open, for sure.

Replace with 195degF thermostat, NOT 180degF.

Radiator is there to get rid of extra heat, so it may not be used at all in very cold weather, and is only used in warm weather after 5 to 8 minutes.

Another FYI, a gas engine needs to run at about 200degF to 220degF to get best MPG and best lubrication

 

Good reply RonD. In adjusting the temp probe of my recent e-fan install, I was trying to calculate the "turn on" temp of the secondary speed of my 2 speed e-fan. In reading your recomended engine operating temperatures for best MPG/oil lubrication, I set the probe temp to 205 (approx) degrees to trigger on the secondary (high speed) fan.