Service Guys Clueless on error P0128

[CoryM]

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.

[werther]

Originally Posted by CoryM

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.

[WeeYari]

Originally Posted by werther

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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[CoryM]

Originally Posted by werther

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.

[dj92]

Originally Posted by CoryM

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.

[Aj353519]

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