Where is the ECT Sensor Located?

Here's the two drawings available for the Avenger 2.0 / 2.4L.

Be aware diagnostic fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or an unacceptable operating condition. When a sensor or other part is referenced in a fault code, it is actually the cause of that code only about half of the time. First we have to rule out connector terminal and wiring problems, and mechanical problems associated with that part before we spend our customer's money on a replacement.

This is especially true for temperature sensors. They have just one component inside them, so failures are very uncommon. By far you'll find more problems caused by corrosion on or between connector terminals, or those terminals are spread causing a poor connection.

Since the Check Engine light turned on, we know the Engine Computer detected the problem. The fault code can get rather specific as to what is wrong with the circuit. Rather than replacing another part that is likely okay, the place to start is by reading and recording that fault code. Once you know the exact code number, you can go here:

https://www.2carpros.com/trouble_codes/obd2

to see the definitions, or I can interpret them for you. If we need to go further, please list that code number in your reply, and expand on the overheating problem. Does this occur at low speeds or with prolonged idling? Does it occur at highway speed? Have you had to add coolant recently or was it just drained and refilled with new coolant? Any excessive white smoke from the tail pipe? Any other recent history that led up to this? Can you hear if the radiator fan runs when the engine is hot?

I'm here almost every afternoon and will be watching for your reply.

The radiator fan shouldn't run until coolant reaches a rather high temperature.

If you're adding coolant but you don't see an external leak, the most likely suspect is a leaking cylinder head gasket. There is a chemical test that can be done at the radiator that can verify this, but it is not conclusive if it shows up negative. This video shows how the test is done:

https://youtu.be/5zpNjYmmiYY

If the special liquid turns yellow, the cylinder head gasket must be replaced. This indicates combustion gases are getting pushed into the cooling system. If the liquid stays dark blue, it only means those gases aren't leaking into the cooling system right now. When that leak occurs, coolant goes through it the other way, gets burned, and shows up at the tail pipe as a sweet-smelling white smoke.

When the coolant level gets low enough, the water pump can't circulate it through the radiator and through the heater core. That's when the engine coolant temperature goes up and the air from the heater turns cold. This occurs first when the engine is idling because the water pump is running too slowly to circulate the coolant. At higher speeds it will still do its job for a little while until the coolant level continues to drop.

Here are links to some related articles you might find interesting:

https://www.2carpros.com/articles/engine-overheating-or-running-hot

https://www.2carpros.com/articles/coolant-leak-symptoms

https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

https://www.2carpros.com/articles/engine-cooling-system

https://www.2carpros.com/articles/radiator-pressure-test

https://www.2carpros.com/articles/white-smoke-or-steam-coming-from-the-exhaust-pipe

This isn't a problem with the water pump. You said you've been having to add coolant to keep the system full. That tells us the level is dropping in the engine, including in the heater core. When that level gets low enough, the water pump is trying to pump air. It can't circulate liquid when there isn't any liquid inside to pump.

When there's no circulation, no hot liquid is flowing through the heater core. That's why the air temperature from the ducts drops. Depending on the design of the entire system, that flow through the heater core might stop first while a little coolant is still getting pushed through the radiator, or it could be the opposite and flow through the radiator stops first. When that happens and there's still a little flow through the heater core, it can act like a small radiator to help keep the engine temperature from getting too high, for a little while.

When the coolant level is just on the edge of being too low, the water pump might still be able to circulate a little coolant when its speed increases. It will be more likely to run hot at lower speeds, such as during city driving.

It doesn't matter whether you lose the heat from the heater first or the engine temperature rises first, it's the loss of coolant that must be addressed. If you don't see any wetness on the engine, check for a puddle on the ground under the engine where you've been parked. If there's no evidence of an external leak, the cylinder head gasket is by far the most common suspect.

Another good suspect, but for an external leak, is the heater core itself. If your car has air conditioning, there will be a water condensate drain pan in the dash, with a drain tube under the hood, on the firewall, near the passenger side. It is normal and necessary for water to drip from that drain tube when using the air conditioning. The liquid will be clear.

If the heater core develops a leak, the coolant will drip into that same drain pan, then onto the ground close to the right front wheel, or right under where the front passenger's feet will be. Leaking heater cores and radiators are more common when the coolant hasn't been replaced periodically. Antifreeze is alcohol which never changes, but it's the additives in it, like corrosion inhibitors and water pump lubricant, that wear out in about two years. It's necessary to replace the coolant to replenish the additives in the system. If the heater core is leaking, the puddle will be the color of your coolant, typically green, or possibly red.

Related to the drain pan, it's not uncommon for the drain tube to become blocked with leaves or other debris. When that happens, once the water level builds up high enough, typically less than an inch, it overflows the drain pan onto the front floor. It can also cause a sloshing sound under the dash. Those wet floor complaints start showing up in hot weather when the AC systems are in use. If a heater core is leaking, and that drain tube is blocked, you'll see wetness any time of year on the right front floor.