Code P0128, possible overheating?

STEWART BRITTS

MEMBER
2011 FORD FOCUS
2.0L
4 CYL
2WD
AUTOMATIC
161,000 MILES

I recently had the check engine light come on; it was code P0128.
When I did research, I discovered that the most likely cause is the thermostat, so I replaced the thermostat.
It was a difficult task in this vehicle but the check engine light went away and everything was going great until I noticed that when I turn off the car, wait a couple of minutes then turn the key enough to light up the instrument lights the temperature gauge was reading 3/4 of the way to hot.
When I plugged in my OBD2 Code reader and used Torque It said the coolant was 240 degrees.

The thing is when I start the engine the temperature drops back to normal the entire time I'm driving or sitting at a light it only seems to overheat when the engine is off.

I did more research and thought there might be air in the coolant system because of the thermostat replacement.

I bought a funnel kit to bleed the cooling system and get the air out but that didn't work.

I'm really not sure what my next course of action should be, I thought about replacing the temperature sensor, but I don't want to waste money I might need if it turns out to be something else.

Any suggestions would be great, and thanks so much in advance for your help

Stewart

Monday, January 8th, 2024 AT 2:03 PM

3 Replies

CARADIODOC

MECHANIC
34,330 POSTS

You're looking at a normal condition we never look at. That being what happens after the engine is stopped. During normal operation, the heat is generated "over there" by the cylinders, but the temperature is measured "over here" by the coolant temperature sensor, or in Ford's case, a cylinder head temperature sensor. At the same time, cooled coolant from the radiator is coming back into the engine. The flow keeps carrying heat to the radiator to be dispersed into the air.

When the engine is stopped, that high temperature is still there next to the cylinders, but now there's no cool incoming coolant to carry it away. Within a few minutes that heat spreads out and migrates over to the temperature sensor where it is seen as increasing temperature. That increased temperature due to lack of flow is what you're seeing on the scanner.

As far as suspecting there's something wrong with the sensor, temperature sensors have an extremely low failure rate because there's just one component inside them. That's a temperature-dependent resistor. By far more problems with that circuit are due to corroded or loose terminals in the connector, or occasionally a cut or grounded wire. The one notable exception, since these are so reliable, is Ford had a huge rash of failures in the early '90s. There had to be a manufacturing problem to affect so many of them.

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Monday, January 8th, 2024 AT 4:20 PM