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Old 06-23-2015, 07:47 PM   #1
werther
 
Drives: Yaris Hatchback
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Question Service Guys Clueless on error P0128

Hi guys!

My first post! came here looking for good advice. Wished my first post was motivated by better reasons... but here we go.

Yaris 2008, 30k miles. Awesome car. Never failed me. Never missed a service. Bought in Mass. now in Texas border town.

Engine light on, checked with the reader: P0128.

Took it to the local Toyota dealership I always go for service. They kept the car 48 hours, told me it was the thermostat and its gasket something. They replaced all that, 300 bucks and off I went. After 24 hours, the same issue again.

This time the had the car for the weekend, and "they found" it was the thermostat cord and it was replaced, they did not dare to charge me.

At this point I am thinking "So, are they just trying different things? taking a shot at fixing the issue? they changed a perfectly working thermostat?"

OK. I drive the car 2 days... and AGAIN!!!

I drop off the car... it has been 8 days... and now they are saying it is something about the cord "don't know what" but something related to the cord.

Total? $1200.

So, I really don't know what to think. Some of you guys drove your Yaris for over 200k km... I can't think of how changing something that sounds so simple like a cord, can be $1200 bucks.

I would appreciate your thoughts/advice on this one. For now, I am not planning on getting it fixed with this guys, they looked to me totally incompetent.

Thanks!

Last edited by werther; 06-23-2015 at 07:48 PM. Reason: added an icon
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Old 06-23-2015, 10:51 PM   #2
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P0128 means the computer is seeing coolant temps below the minimum (whether it is or not). 99% of the time this means the thermostat is stuck open, and the engine warms up too slowly. Tsats sometimes act up intermittently, so I don't blame them for starting with a Tsat even if it tested out OK in the shop. It's step 1 for sure. Since it seems the Tstat is not the problem, now it gets a little harder (especially since it isn't happening all the time). It could be: faulty sensor/wiring, faulty ECU, faulty ECU software, low coolant (leak), incorrectly mixed coolant, waterpump not circulating and a few other things. I know it's frustrating as a customer, but unless the problem happens quickly so you can test it while it's acting up, it can be very tough to diagnose.

I am not sure what "cord" they are mentioning. If you could get a better description, I may be able to advise further. $1200 goes a long way on a Yaris so I am curious what they are wanting to do. Possibly a wiring harness?

Cheers.
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Old 06-24-2015, 03:13 AM   #3
dj92
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Get yourself an OBD adapter and watch how the measured coolant temperature develops during warmup.
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Old 06-24-2015, 06:20 AM   #4
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Ideally you get a code reader and tell us what the code is. Then we tell you what it is and you do the work. Repeat until fixed.

Doing this kind of work at a shop out of warranty SUCKS!

Most of the time driving with the CEL on doesnt hurt anything. Would this be the case in this instance?

Its getting so expensive to work on cars that a fender bender can total the car. Now maybe the same thing will happen for a CEL!
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Old 06-24-2015, 07:14 AM   #5
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Get an ultragauge offline for cheap and use it to see if it correctly reading the coolant. If it is not than you know you just need to replace the coolant temp sensor.

One thing you will want to look at it your oxygen sensors. On Toyota's a diconnected/ faulty pre cat converter oxygen sensor will trip that code. Mine got partially diconnected and it recently tripped the code. Put dielectric grease on the connector and properly connected it and it fixed the issue. I also have an ultragauge so I could tell that the temp sensor was good (it was also recently replaced).
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Old 06-24-2015, 08:09 PM   #6
werther
 
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Thumbs up

Quote:
Originally Posted by CoryM View Post
I am not sure what "cord" they are mentioning. If you could get a better description, I may be able to advise further. $1200 goes a long way on a Yaris so I am curious what they are wanting to do. Possibly a wiring harness?

Cheers.
First off thank you all for the suggestions/opinions. I appreciate it.

So I went back to the dealership/service center. We sat with the manager and the technician.

Basically they went step-wise changing parts that trigger P0128. As follows:
- Tstat
- Cooling system
- ECT sensor

Next in line...
The WIRE HARNESS

They walk me through those steps. Yes, CoryM that is what they are planning to do.

They showed me the factory white page of the wire and it is a hell of a job for such an error that a priori seems so innocent, I mean, it isn't that the car overheats or can't run!

Anyways... we reached a deal for parts cost only which I seemed fair.

This made me wonder of you guys driving your Yaris for over 100k miles... how much of this stuff you have to (or I will have to) go though only to keep this beautiful car.
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Old 06-24-2015, 09:01 PM   #7
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Quote:
Originally Posted by werther View Post
This made me wonder of you guys driving your Yaris for over 100k miles... how much of this stuff you have to (or I will have to) go though only to keep this beautiful car.

Sadly, your situation seems to be one of those 1 in a million cases.



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Old 06-24-2015, 11:51 PM   #8
CoryM
 
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Quote:
Originally Posted by werther View Post
First off thank you all for the suggestions/opinions. I appreciate it.

So I went back to the dealership/service center. We sat with the manager and the technician.

Basically they went step-wise changing parts that trigger P0128. As follows:
- Tstat
- Cooling system
- ECT sensor

Next in line...
The WIRE HARNESS

They walk me through those steps. Yes, CoryM that is what they are planning to do.

They showed me the factory white page of the wire and it is a hell of a job for such an error that a priori seems so innocent, I mean, it isn't that the car overheats or can't run!

Anyways... we reached a deal for parts cost only which I seemed fair.

This made me wonder of you guys driving your Yaris for over 100k miles... how much of this stuff you have to (or I will have to) go though only to keep this beautiful car.
Wiring harness issues are not common with this car, but can happen on any. Especially if the car has been in a crash or had work done where the harness may not have been correctly routed/secured. Just throwing it out there, but in that entire engine harness, there are only 2 wires that go to the sensor from the computer. In some cases, I have just replaced the required wires, and routed the new ones on the outside of the harness. It's not ideal, but is much more cost effective. No question that replacing the harness is better, but if the two wires are done correctly you will still correct the issue.

Cheers.
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Old 06-25-2015, 06:05 AM   #9
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Sounds like they tried to do right by you.

Almost no one has problems like this here. These cars have become so complicated that once itna while extraordinary efforts have to be made to fix them.

Personally I sincerely doubt the wiring harness has anything to do with it. Did they replace the coolant temp sensor?
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Old 06-26-2015, 06:38 AM   #10
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Quote:
Originally Posted by CoryM View Post
Especially if the car has been in a crash or had work done where the harness may not have been correctly routed/secured.
^This.
The easiest way to check the wiring is unplugging the connector from the sensor and measuring the voltage across the two pins while the ignition is on.
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Old 05-08-2018, 07:13 PM   #11
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same code....not the only one

the thermostat has been replaced and the coolant temperature sensor. i have reset the ecu and cleared the code with the code reader twice...check engine light is back on with the same code. 2008 yaris sedan
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Old 05-09-2018, 06:44 AM   #12
dogsridewith
 
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Mind of Toyota error code replace wiring harness Yaris connector claw

Replace whole wiring harness is ridiculous.
Tight Toyota connectors, no two the same, w/ no adequate diagrams of where the mystery release hook is and which way to move it. Incites perfectly good mechanics to destroy perfectly strong connectors by prying and yanking them apart.

(and read the thread here where Mind of Toyota had to send a pair of monolingual Japanese techs flying around the world to inspect enough individual cars to figure out that one particular one had the wrong wiring harness.)
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Old 05-10-2018, 12:39 AM   #13
CoryM
 
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So step one is to find out whether it is mechanical or electrical. You will have to measure the temperature of the coolant and see if the engine is actually running cooler than it should. IIRC the Tstat is 180F and the code comes on if it sees the engine at 165F. Compare your actual coolant temps with the temps the sensor reads and see if they are the same.
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Old 05-11-2018, 01:52 PM   #14
B.B._07
 
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With the thermostat and ECT sensor both replaced the only other causes are low coolant level, the wiring back to the ECU, and the ECU itself.

I'd start with checking the coolant level in the reservoir, and in the radiator. Are you getting good heat in the car? If not there might be an air bubble in the cooling system that could be the cause of both issues. The ECT sensor is a simple thermoresistor or thermistor that changes resistance valve with changing temperature. Check the connector at the ECT and ECU for any corrosion, damage, etc as well as the wire harness for frayed sections or rodent damage. Any of those things could cause additional resistance which would result in the ECU getting a faulty temp valve.
Attached Files
File Type: pdf Yaris_P0128.pdf (34.4 KB, 5 views)
File Type: pdf Yaris_ECT_Sensor.pdf (178.4 KB, 4 views)
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