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Discussion Starter #1
I got this unexpected code today on my wife's car. 2012 Sonata 2.4, about to hit 180K.

She left the house this morning and drove 30 minutes to an appointment, and that's when she noticed the check engine light. The most common cause I'm seeing for this are a thermostat stuck open. It so happens that I replaced the thermostat 2-1/2 months / 2,000 miles ago. I used a genuine Hyundai stat & gasket, and put in new coolant. She sent me a pic of the dash which clearly shows the temp gauge at normal operating temperature. So I'm pretty confident the stat is working correctly.
442173


The next possible cause I see is the temp sensor not working. This raises the question, is this sensor different from the one for the gauge pictured?

If anyone has any diag advice I would appreciate it.
 

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Why did you change the thermostat 2 1/2 months ago? Is the indicated temperature of both thermostats the same?
 

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Discussion Starter #3
I changed the stat because it had been in there for 140K / 6 years. I usually change them at 100K because I've had stats lock up on me over the years. This one was in longer than usual because I got lazy.

The stat p/n is 25500-23010 which is the OEM stat, I don't know what the temp is, but I believe it's 180F. No issues with it over 2K miles.
 

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I would assume it's not the ECT sensor since the correct temp shows on your guage, but some googling indicates it could be the IAT sensor perhaps? Someone with more knowledge on the subject might be able to elaborate on this.
 

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I would assume it's not the ECT sensor since the correct temp shows on your guage,

Good assumption.

but some googling indicates it could be the IAT sensor perhaps?

Not at all. Whoever told you that has no idea of what they are talking about.

Someone with more knowledge on the subject might be able to elaborate on this.
Could be a simple glitch. Can you clear the code and see if it resets?
 

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Discussion Starter #7
I cleared the code, will see how things go. Thanks for the replies...always interesting figuring these things out!
 

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She sent me a pic of the dash which clearly shows the temp gauge at normal operating temperature.
While P0128 triggers if it takes too long to warm up, the engine can eventually reach normal temp after the code is set.
It's possible the new OEM 'stat is defective.
I suggest graphing the temp during warmup with an OBD scanner.
You can also remove and test the 'stat with a pan of heated water and a thermometer.
 

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I can't see what you've circled, and I don't have one in my hand...a small by-pass valve perhaps?
 

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If the temp doesn't come up to 185F or so with a new OE tstat there is a problem I would say. Changed both my cars last year and they both run at 185-187F.

I use the Accent in the winter, it sits outside and my location is rural. So it takes forever to warm the little thing up.

Went to a 190F t-stat and get much better heat. But it still took forever to hit that. And forget about sitting idling as it would never get over 140F or so when around 0F.

Plugging the jiggle valve thou cut the time in half that it takes before the t-stat initially opens.

An OBDII scanner is great to monitor the engine temp. I run one all the time.

The one on the left is an OE for the MC Accent and the one on the right is the Stant 195F. But I just cut the stem off the little ball and a small machine screw I had fit perfectly threading in. A bit of loctie a to keep it there.

 

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Discussion Starter #16
I fired up the Sonata this morning and took it for a ride. The temp gauge on the dash went right up to about halfway where it normally does, in the expected amount of time. It then dropped down a bit as would happen when the stat opens up and cooler coolant runs through. The stat should be closed by default, so unless it somehow got stuck open, I can't figure out how I got a low temp condition. I only had a short trip today, no check engine light. If it happens again - which I kind of expect - I'll go from there.

That little doo-hickey on the OEM stat is a bleeder valve and they want it positioned at 12 o'clock.
 

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Discussion Starter #18
I replaced the thermostat with another new OEM stat. After a few trips out the P0128 code is back.
 

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I replaced the thermostat with another new OEM stat. After a few trips out the P0128 code is back.
Try replace the temp sensor or check the connection first. The thermostat and sensor are in same general area. It is possible the Sensor connector is not snugged or knocked loose when you replaced the thermostats.


What caused my vehicle to set a P0128 trouble code?
The engine control module uses the Engine Coolant Temperature (ECT) sensor to determine whether or not to trigger a OBD-II P0128. When a trouble code P0128 exists by itself, usually a shop technician will suspect a fault engine coolant temperature sensor or stuck open thermostat.
  • A stuck open or prematurely opening coolant thermostat.
  • A defective engine coolant temperature (ECT) sensor and/or ECT wiring (most often at the ECT connection/harness).
 
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