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Q: VCR and Engine Light On, DTC P0128 on 2004 Toyota Highlander

Hi Pat Mannion,

VCR and Engine Light On, DTC P0128. A shop said to replace the thermostat. Their data had it opening 176-183 F, maybe it is not completely closing. They also recommended replacing the ECM and maybe the ECT sensor. What do you recommend?

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Hello Bob,
ECU replacement is certainly not necessary. P0128 is for low engine coolant temperature reading. A repair shop should plug in a scan tool to the 16 pin diagnostic socket under the dash and read stored freeze frame information to see if the car was in closed loop and what the temperature was when the Check Engine Light first came on. Then run the engine and look at live sensor information and see what the computer sees as the actual running temperature. If the temperature is below 175 degrees, it is most likely the thermostat is stuck open, replace the thermostat drive the car check to see if the temperature gauge now reads toward the middle of the gauge, if it now does or you found the thermostat stuck open you can be pretty confident you have fixed the problem. I have replaced a few coolant temperature switches to correct this problem. The sensor is on the passenger side top of the engine the two wires attached to it are green/black and white/black.
Hi Pat,

I believe the shop did this. I have an Alldata graph with ECT on the vertical axis and time on the horizontal, a horizontal threshold line at 167 F, estimated ECT graph line that goes above the threshold and an indicated coolant temp. graph line that is well below the 167 F threshold. An arrow points to 5 seconds after the Estimated ECT graph line passes the threshold it is labeled DTC set (after 2 driving cycle).

It appears to me that the thermostat is stuck open. I expect it would be easy and inexpensive to just replace it. If this solves the problem will the error code go away or will it have to be manually reset?

Heading off to Jackson Lake with the boat for camping and fishing this weekend. Thanks for the car info.

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