Coolant sensor is bad on my 01 Eclipse 2.4 L motor. I need to know where it is on my car and how to change it

Here are a couple of thoughts on this problem:

1. Just for fun and giggles, it might be a good idea to have the thermostat checked one more time. I know it is a new thermostat but you never known if a thermostat may have failed even right out of the box. It is possible that thermostat could be shot, even thought it was new. I know it is a long-shot, but it never hurts to give it a try.
2. Check the fan sensors. The Eclipse has used an electric, rather than mechanical, fan for some time. It is possible over time that the start and stop sensors can go out of tune and that they can raise your fan's run rate on starting, cruising and idling. In this case, the mechanic is right in that the higher speed is causing the coolant to be cooled off more quickly than it should and the result is that your vehicle's cooling system is causing problems for your Eclipse's heater sensors. If the sensors don't get information that the coolant flowing through their portion of the cooling system isn't the right temperature then the heater won't kick off and all that will happen is that the heater will wait until it senses the coolant is the right temperature.

Possibly, it all depends on the ECU and engine sensors. The only way is to get a workshop manual and check all the connections. Mitsu often change sensors and connectors between years.

Assuming the cooling system is full of fluid, there is a coolant temp sensor that tells the computer what the engine temp is. When the engine reaches aprox 220 degrees the computer grounds the fan relay.
If you have tested the fans and you know they work, could be the sensor or the relay, or the computer.

did you change the powertrain control module with the trans?? if not thats probably where the problem lies, ive saw many people get mad and agervated just to realize they had to change the computer for trans with it..... just a thought.

It could be the radiator fan motor if all sensors and relays have been changed.......unplug the coolant temperature sensor the fan should come on and stay on unless the motor is bad or it is not getting electrical current.

Your heater blower, inside the car works to provide heat to the interior of your car, and the air is hot, right? If the cooling fan is not coming on, depending on the outside ambient temperature, you may be using your heater coil for the heater to dispell suficient heat (inside the car), combined with a low outside temperature to keep the engine at normal operating temperature negating the need for the cooling fan. OR your coolant temperature sensor is bad, and the ECU doesn't know that the engine is hot and to turn on the fan. This sensor works by changing the voltage returned to the ECU depending on the temperature of the tip, which changes the resistance to the computers inquiry. So, the computer knows the temp of the coolant. It uses this information to adjust mixture gauge etc. Most now have 2. One for the gauge, and another for the ECU. So don't go by the gauge working. Then there is the fan motor itself. This can be tested by using a testlight on the wires when the engine temperature gauge in at 3/4 to the hot side of the gauge. The other way to test is to bring a live wire to the motor. If the motor is good, the sensor is bad, if the motor is bad, well the motors bad. I hope this helps you, write back if you need additional assistance. . Please take the time to rate our free service, and my performance. Thank you for trying FixYa.com!

We just had this and it was a sensor called something like the CAM TIMING SENSOR - which gave the engine bad timing when it tried to get going. The car is great since we got it replaced - but it cost like $360 to get it all diagnosed and replaced.

THERE IS A PROBLEM WITH THESE SENSORS, IF THE COOLANT ISN'T LOW REPLACE THE SENSOR.

Your question was truncated. I see that the bad sensor is on bank 1, but I can not see if it is the upstream (sensor 1) or downstream (sensor 2) sensor. The upstream sensors are mounted in the exhaust manifolds (look sort of like spark plugs) and have wires going to them.
You will need a 7/8" or 22mm wrench to remove the sensor (after disconnecting the wires). If your car has over 70,000 miles I STRONGLY suggest that you replace BOTH upstream sensors. One in each exhaust manifold.
Charlie

its screwed into the cylinder head ,when you fit a new 1 dont overtighten it they sense temp of the head they dont screw into the water jacket!!!!!!!!!!!!!!!!!!!!