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Discussion Starter #1
After an almost catastrophic transmission fluid leak last month (replaced radiator and got this fixed), my Pilot is now giving me CEL reading P0128. Any suggestion is much appreciated, please! Thanks in advance! Also, hopefully someone could please tell me how to replace it. Thanks again!
 

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Check the coolant level. Bring the car to temperature and idle. Use a handheld temp reader and shoot the hose attach point where the coolant leaves the engine. use your scanner to read the temp. If the two match within a few degrees, then thermostat probably need changing. Thermostat might be old or the spring old and let it open too soon.
 

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Discussion Starter #4
Ok, thanks! I don't have the handheld temp reader. So, I ordered the parts suggested since that's the likely culprit (thermostat replaced with timing belt at 100k. mi and it's only 140k mi now).
 

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Did you burp the system properly? I am not aware of the Pilot being prone to air pockets, but everything is possible. Do you get normal level changes from min to max in the expansion tank as your engine warms up?
 

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Discussion Starter #6
The mechanic changed the radiator and coolant. I've been driving it for almost a month now for 15 miles at a time and back (to my wife's favorite shopping center). Thanks.
 

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In that case open the radiator cap when cold, check the level and top up if low. Start the engine and let it warm up. Get your wife to rev the engine to 1500 RPM and you should see the coolant flow in the radiator. If you do not see the flow, rev the engine to 2000-3000 RPM. The should be a surge and some coolant would spit out of the mouth of the radiator.
 

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Discussion Starter #8
Good suggestion! Hmm, shouldn't the CEL happened already earlier after the new radiator? The radiator and the reservoir has correct level as of yesterday.
 

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Discussion Starter #9
Ok, I did re-burp it, just to be sure! Topped it off several times. Drove it around the neighborhood right after. CEL is still on. Waiting for the part to arrive tomorrow... Thanks!
 

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Hopefully its a breather hose. I’ll take a look at mine and see if it’s the same.
 

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I've seen two conditions of these breather hoses: clean like in the picture above and in my Pilot, or with a chunk of greasy dust at the end, and nothing in between. I wonder if the greasy dust accumulation is due to the milk shake foaming and rising up the hose until the fluid is changed.
 

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Or could be from a slight amount of fluid escaping when the tranny overheats. I looked at mine and it’s pristine. And the hose is hidden under the engine cover so I never knew it existed before today.
 

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Discussion Starter #16
Thanks! Never noticed it before. The only two times I removed the engine cover were the times I replaced the spark plugs about 8 years ago and last month when I attempted to replace the radiator.
 

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Discussion Starter #17
So, the new sensor just installed. I cleared the code. Drove it for 15 min. No CEL. Incidentally, I read this instruction on how to diagnose stuck open and stuck closed thermostat. Here is the instruction:
Can you test a thermostat without removing it? Mechanics test the thermostat by measuring the temperature of the upper and lower radiator hoses with an infrared thermometer, while monitoring the engine temperature with a scan tool. See the chart below.

Mechanic measures radiator hose temperature with an infrared thermometer
Mechanic measures radiator hose temperature with an infrared thermometer
Thermostat opening temperature chart

The engine warms up at idle. The yellow line is the engine temperature. The thermostat opens at 185°F (85°C) and the temperature of the lower radiator hose (blue line) becomes almost the same as the temperature of the upper radiator hose (red line).
In this chart, the yellow line is the temperature of the engine, the red line is the temperature of the upper radiator hose and the blue line is the temperature of the lower radiator hose.
When the engine is started cold, the upper hose warms up with the engine, but since there is no flow through the radiator, the lower radiator hose remains cold.
In this Honda in the chart, the thermostat starts opening at about 185°F (85°C). As the thermostat opens and coolant starts flowing through the radiator, the temperature of the lower radiator rises quickly close to the temperature of the upper hose. At about 204°F (96°C), the radiator fan comes on and the temperature drops a few degrees. The thermostat in this car works properly.

If a thermostat was stuck open, the lower radiator hose would start warming up as soon as the engine is started.
If a thermostat was stuck closed, there would be no flow even after the engine reached the operating temperature, and the lower radiator hose would remain cool.
Of course, lack of flow through the radiator can be caused by many other reasons besides a stuck-closed thermostat.
If a thermostat is stuck closed, the engine might overheat, which can result in expensive repairs. If there is a suspicion that the thermostat is stuck closed, it's usually replaced.
 

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Discussion Starter #18
"As the thermostat opens and coolant starts flowing through the radiator, the temperature of the lower radiator rises quickly close to the temperature of the upper hose.". Hmm, there is a caveat to this--after I drove the Pilot for 30 min (20 miles) on back roads and highway speeds, the temp highest reading was 183 on the scanner. The upper hose was very hot that I could barely touch it for a second or two! However, the lower hose remained just warm to the touch--I was able to touch it for as long as I wanted to. I also performed the "squeeze test" for thermostat being stuck closed. With the engine running, I squeezed the upper hose (with mitten) and it was soft and squeezable (if too firm and unable to squeeze, the thermostat is stuck closed). I drove the Pilot with the ac on, one eye on the temp gauge and one eye on the CEL (not there!). So, emission testis next! Due this month. Thanks!
 

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Discussion Starter #19 (Edited)
Oh, II just realized that thermostat is not automatically offered by dealer for replacement with timing belt along with water pump! My record shows that timing belt, water pump and drive belts (I requested this) were replaced at same time in 2011 at 97k miles. Had I known that code PO128 has thermostat as the primary source of the CEL, I would have replaced the thermostat, instead! And it seems thermostat is working (not stuck). Oh well, coolant temp sensor is now replaced and the CEL is gone (for now?). Does anyone here had their thermostat replaced with timing belt?
 

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Standard dealer timing belt job is timing belt, water pump, and coolant. Add-ons appear to be tensioner, cam seals, and serpentine belt. I could see the thermostat and hoses being suggested to change during a radiator replacement,

And the lower radiator hose should be cooler than the upper after and extended drive since the radiator is removing heat from the coolant.
 
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