P0128 Thermostat fail monitor will set a DTC usually when the thermostat or cold start monitor fails because the actual engine temperature is less than what the PCM expects.
But this is not true all of the time; this is what I use to think, I'll explain later.
If the ECT "Engine Coolant Temperature Sensor" signals to the PCM the engine is cold, a P0128 Thermostat monitor failure will occur.
There will be an associate freeze frame, DTC "Diagnostic Trouble Code" and check engine light on the dash.
The most common corrupt is when someone removes the thermostat, changes the thermostat with a lower temperature one or the ECT sensor is defective.
The CCM "Comprehensive Component Monitor" will monitor the temperature, of the engine for several minutes and if the CCM is detecting a cold engine, this wouldn't be rational, and the thermostat monitor will record a failure.
Other failures could be a stuck open thermostat, leaky coolant somewhere in the system.
I also saw when the ECT "Engine Coolant Temperature Sensor" was defective indicating a cold engine causing a P0128 code.
Now I had a Toyota 4 runner with a DTC P0128, so the class and I replace the thermostat, flushed the coolant and readded antifreeze with a 50/50 mixture.
Thinking we fix the car we cleared the codes and gave the vehicle back to the customer, guess what the next day the car was back with a DTC and a check engine light with a DTC P0128.
We look for other leaks and there were none, the vehicle was up to operating temperature, but the code was there, it had to be a different failure.
The problem was ready for this, a defective O2 sensor or on this case AFS "Air Fuel Sensor" and that was the fix.
How I check it AFS, was by creating a rich or lean condition and watch the voltage on the scan tool, the sensor voltage didn't move it just stayed fixed.
Keep this in mind an AFS doesn't move like a Zirconia O2 sensor, but it still is supposed to move when the vehicle is fully rich or lean.
We replace the sensor, "and I was worried about this one" we cleared the light kept the vehicle for a few hours drove the car no light.
So we gave the customer his truck back, and the light never came back on, it was a successful fix.
So when someone says P0128 is always a thermostat well that's not true, in least in this case it wasn't.
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