New temp sensor, same problems

welcome!
were you having the heater problem before changing the sensor?

watch

engine-coolant-temperature-sensor-replacement-p0117.230147

 

Your problem may be the engine coolant water pump or the thermostat. My bet would be the water pump.

How many miles on your Prius?

 

If there are no codes other than P0117, the most likely candidate would be a short in the ECT sensor wiring harness. And given that the problem has persisted across two sensors (neither of which needed to be replaced without first ruling out the harness), it is even more likely.

 

The ECT is simply a temperature sensitive variable resistor. If you still got the old ECT; get some boiling water and hook it up to an ohm meter. Dip the ECT into the boiling water - there should be a resistance change. The water temperature reading should correlate to the engineering specifications of the ECT. If the ECT it good, it can only be a wiring or ECU issue.

 

Imagine for a moment that your hand has just lost all sensation, and you dip it into a pot of still water. If the water happened to have been at room temperature, no harm done. Different results entirely if the water had been boiling a minute ago, but of course you can't immediately tell because there is no feedback from the temperature sensor (your skin).

With P0117, the car is in a similar situation because the coolant temperature sensor is not reporting anything. The ECM just assumes the coolant temperature to be a fixed 80 °C, but that assumption may be very far off the mark and lead to an engine overheat, and then you're looking at a repair that is may be a couple of orders of magnitude more expensive.

P0117 specifically reports that there is a short somewhere in the ECT sensor circuit. The only three places that the short could be in are the sensor, the wiring harness that connects it to the ECM, and within the ECM itself. Your working assumption was that the original sensor went bad, so you replaced it with a new (aftermarket) one. Why do you still think it is the new sensor that is faulty and not the wiring harness?

 

It wouldn't be the first time I've gotten a bad new part - that's why you need to test, rather than just throw parts at it. There's also a lot of counterfeit parts floating around out there, kinda like those mis-spelled brand name stuff you find at the dollar tree or flea market..

Just saying...

 

How long have you had, since new, recent acquisition, somewhere in between? Intake manifold and EGR been cleaned?

 

Let's start back at the beginning:

You are not telling us the full story. Why did you replace the engine coolant temperature (ECT) sensor? Did the high temperature light come on and also a check engine light? Did you scan for trouble codes before changing the ECT sensor?

How does the ECT sensor work? It is in contact with the engine coolant and the resistance of the sensor changes when the coolant temperature changes. As the engine coolant increases in temperature, the resistance of the sensor decreases. See post below made by the very knowledgeable ChapmanF:

Also ChapmanF wrote, "The ECM will log a P0117 if the ECT resistance gets so low it looks shorted. But that would correspond to a coolant temperature above 284 ℉ (140 ℃)."

So here is my theory what is happening with you Prius.
The engine is overheating. The high temperature light comes on. The P0117 trouble code is triggered because the engine coolant temperature goes higher then 284F. The check engine light turns on because of the P0117.
The ECT sensor is working correctly and did not need to be replaced.

So the main question is why does the engine overheat? My guess is that the engine coolant water pump is going bad and needs to be replaced. So this post is the "full story" which I did not have the time to write in my post #3 above.

In post #5, you said that you have 150k miles on your Prius. Water pump problems usually occur at 150k and higher.

I did some searching on this forum for similar problems that you are having with your Prius and found the following posts:

 

I would interpret it differently: if you follow a graph of the ECT sensor's resistance, you see that coolant would have to hit 284 ℉ (140 ℃) before a normally-functioning sensor would look like a short circuit.

But what that's really telling you is that a normally-functioning ECT sensor really isn't ever going to look shorted, because 284 ℉ is well above the boiling point of the coolant at the pressure limit of the cap. If the coolant got close to that temperature, the clouds of sickly-sweet steam billowing out of the engine compartment would get your attention before a check-engine light would. (With any luck, you would already have noticed the red overheat light coming on at 248℉.)

So in my interpretation, I would lean more toward P0117 likely indicating that there really is a short circuit somewhere, such as some insulation scraped or chewed off the sensor wiring somewhere. Such a problem, of course, would likely still be there after putting in a new ECT sensor.

 

Before considering replacement, you would still want to verify that the fault exists in the wiring harness before you go about replacing a $1000 cable. D6 is the ECT sensor connector, D28 is the larger ECM connector, and you need to check for continuity between D6-2 and D28-64, and between D6-1 and D28-65. Check also that there is no continuity between D6-2 and body ground.

Only one branch of the wiring harness connects to the ECT sensor. The same harness (part no. 82121-47111, colored blue in the image below) also connects to multiple other sensors and actuators in the engine bay, and replacing the whole thing is, as one might imagine, no walk in the park. It may be easier to repair the harness in place, but you'd first need to determine the location of the damage by visual inspection.



And if you were inclined to actually replace the entire harness, a local salvage yard or the Toyota Parts website are good places to start.