P0128 Coolant temperature below thermostat regulating temp

[Bambam234]

Got a check engine light yesterday and pulled the code P0128. It’s a 2014 DE. I bought the car used with 80k and can tell maintenance wasn’t in the previous owners budget. I have already replaced water pump, temperature sending unit, and thermostat(it’s hot in South Carolina and I wanted to make sure it was going to be dependable). My scan tool will give a list of recommendations that could cause any fault. Inlet air temperature sensor was listed after Thermostat, water pump, and coolant temperature sending unit. After a lot of searching found that the Inlet air temperature sensor and the Mass Air Flow sensor are built together on the mirage. Just posting this in case it helps anyone else.

[Fummins]

Was there a reason you replaced the water pump, thermostat and temp sensor? Those aren't parts that normally need to be replaced in these cars, at least not in these parts. Were they replaced with oem parts or the finest quality Chinesium?

Did the code appear after or before you replaced everything? If after is there air in the system? Does it take a while to get heat from the vents, how long til the green temp light goes out?

[Bambam234]

Parts were replaced because they were bad. Pump was leaking when I bought it coolant wasn’t blue it was more of a mud water look to it. Coolant temperature Temperature sending unit and thermostat was replaced because I was already flushing the coolant. I driven about 5-600 miles since the parts were replaced. All parts have been replaced with OEM with the exception of the thermostat(couldn’t find an OEM thermostat till a couple days ago.

[Top_Fuel]

...how long til the green temp light goes out?

Yeah...what he asked.

If you don't have any way to monitor the real coolant temperature (like a Scangauge), the green coolant light (that comes on when the engine is cold and goes off at 137 degrees...I think) is the closest thing we have to go on.

[7milesout]

...when I bought it coolant wasn’t blue it was more of a mud water look to it.

Uh ohhhhh. Bambam, keep inspecting your coolant. When coolant looks like mud water or a chocolate milk kind of look to it, it usually means a blown head gasket. But like I said, keep watching it. I'm no expert. If the chocolate milk comes back you've got problems (head gasket). Let's hope it doesn't come back.

Fummins weigh-in if you don't mind. Am I wrong?


7milesout

[Bambam234]

Yeah I know all to well blown head gaskets. That why I was servicing the coolant system to start with. I did a compression test (didn’t record values but wasn’t to low for me to believe a blown head gasket. But I will keep an eye on it.

[Bambam234]

Ok so update. Yesterday leaving work car struggled to get up to 68 and would go any faster and actually as I drove kept slowing down. Check engine light came on same code and another code for malfunctioning MAF sensor. After hours of back and forth with orielys and autozone finally found a replacement ($180 later). Installed it last night and drove to work this morning and it drove and ran like a completely different car and no CEL. Next up is finding out what the vibrations I feel from 65-75 but that will be another post for another day! I do appreciate everyone taking the time to comment and try to help

[Bambam234]

My apologies for the late reply. I do have a scan tool it’s a harbor freight one nothing fancy but definitely missing some key data points that won’t track like coolant temperature. I will eventually get another scan gauge to keep in the car or at least handy.

[Fummins]

Ok so update. Yesterday leaving work car struggled to get up to 68 and would go any faster and actually as I drove kept slowing down. Check engine light came on same code and another code for malfunctioning MAF sensor. After hours of back and forth with orielys and autozone finally found a replacement ($180 later). Installed it last night and drove to work this morning and it drove and ran like a completely different car and no CEL. Next up is finding out what the vibrations I feel from 65-75 but that will be another post for another day! I do appreciate everyone taking the time to comment and try to help

Thanks for posting an update. Sounds like you got it figured out.

[Christopher_B]

That is interesting. I would not have initially thought a MAF would cause that but after reading into the code description I can see that the ECM uses the IAT (built into the MAF) to estimate what the coolant temp might be in certain cases. That being said a bad MAF could disrupt the check conditions sequence and cause the estimated coolant temperature to be higher. This probably affected A/F ratios and killed power and mpg.
The following is from ALLDATA but I believe they have a misprint in the line stating "intake air temperature is subtracted from intake air temperature..."
My guess would be the ECT is subtracted from the IAT is a difference of less than 10 degrees C. This would be a cold soaked engine where it is close (within 10 degrees C) of ambient temps. Again a guess and interested in others thoughts, especially those with repair experience.


Check Conditions

- Engine coolant temperature is between -10°C (14°F) and 60°C (140°F) when the engine is started.
- Intake air temperature is more than -10°C (14°F).
- The intake air temperature subtracted from the intake air temperature when the engine is started is less than 10°C (18°F).
- The total time when the amount of intake air is small is less than the specified time.
Judgment Criterion

- After the estimated engine coolant temperature rises above 77°C (171°F), the actual engine coolant temperature is less than 77°C (171°F) even though the specified time has passed.