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Coolant issues

i’ve replaced my water pump and thermostat after getting code P0128 for coolant undertemp and having a small coolant leak, code cleared but came back. While monitoring my obd2 scanner my thermostat opened up at the indicated 220 degrees, what do you guys think? bad new thermostat or faulty coolant temperature sensor? Thermostat was rated at 187 I believe maybe a little lower

The thermostat temperature spec indicates the temperature at which the thermostat opens.

If a lower temperature spec thermostat is installed, it can create that code.


How would you know what temperature the thermostat opens using an obd2 scanner? It seems like the only thing the scanner would tell you is what temperature the coolant is as measured by the engine’s coolant temperature sensor.

Since the scanner uses the same coolant temp sensor to read the coolant temp as the ecm does, if the scanner’s reported coolant temperature is correct, the engine’s coolant temp sensor must also be correct.

I was bleeding the air out of my coolant system and looking at what the sensor was reading. I can tell you with certainty the thermostat opened up while it was reading 220

I think I bought an OEM spec thermostat at the time it was a bit ago so I can’t be certain

If there is air in the cooling system there can be a delay in opening of the thermostat, the thermostat must be submerged in coolant to open promptly.

Don’t bleed air from the cooling system when the engine is above about 205 F, the coolant will boil creating more air.

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What would you suggest? Bleeding it multiple times before it reaches high temperature?

With a bleed screw on the engine you should be able to fill the cooling system when the engine is cold. Open the bleed screw, add coolant though the fill location until coolant comes out the bleed screw, close the bleed screw and fill to the “full” level. Start the engine and operate at 2500 RPMs for 30 seconds to circulate coolant though the engine and heater core. Stop the engine and open the bleed screw and fill the cooling system again. After road testing the vehicle check the coolant level.

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Thanks I’ll give it a shot