Service Engine Soon with P0128 Code

[Z4MC Junkie]

I have a 2007 Z4 M Coupe with 120K miles. A few weeks ago the Service Engine Soon console light came on. I've checked the fluid levels (i.e., oil, coolant, brake and steering) and they are all good. My code reader reports a P0128 (Coolant Temperature Below Thermostat Regulating Temperature) which, based on my internet research, means I need a new thermostat and/or have a bad temperature sensor. I'm reluctant to take it to my BMW guy since I don't notice any engine performance changes, the operating temperature is normal (~210 F), my mechanic typically charges $500+ any time he diagnosis/fixes anything and I'm on the tight side

Any recommendations as to what I should do?

Thanks!

[Clearancediver]

Clear code and see if it comes back….

If it clears then all is good. If not then switching temp sensor might be an option to see if it was that part. Obviously cheaper than paying $500 hr if you have the mechanical smarts.

[bwright657]

I had the same code earlier this year. The car was taking too long to get up to operating temperature once the daily temps here in SC dropped below 45F. No problem keeping the car cool once things got up to temp though.

Doing a deeper analysis with INPA found 2 things. My thermostat was not fully closing and one of the coolant temp sensors was getting bad readings. It read 140F after the car had cooled overnight and the ambient temp was 40F. All of my parts were original, so I decided to replace the water pump, both temp sensors and the radiator hoses while I was in there. No sense in putting 14 year old rubber hoses back on the car when they had to come out anyway to get to the thermostat.

[elerner61]

Originally Posted by bwright657

I had the same code earlier this year. The car was taking too long to get up to operating temperature once the daily temps here in SC dropped below 45F. No problem keeping the car cool once things got up to temp though.

Doing a deeper analysis with INPA found 2 things. My thermostat was not fully closing and one of the coolant temp sensors was getting bad readings. It read 140F after the car had cooled overnight and the ambient temp was 40F. All of my parts were original, so I decided to replace the water pump, both temp sensors and the radiator hoses while I was in there. No sense in putting 14 year old rubber hoses back on the car when they had to come out anyway to get to the thermostat.

Where is the second coolant temp sensor? I know about the one that is plumbed into the lower radiator hose.

[bwright657]

Originally Posted by elerner61

Where is the second coolant temp sensor? I know about the one that is plumbed into the lower radiator hose.

The second one is in the engine block. Behind the oil filter and below the intake manifold.

https://www.pelicanparts.com/techart...eplacement.htm

[Z4MC Junkie]

Thanks all. I erased the codes which turned off the Service Engine Soon light. If the P0128 code comes back then I may just ignore it since the car operating temperature hits normal (~210 F) pretty quickly and the impacts of driving with a cold engine (i.e., lower miles per gallon and potential carbon build up) are not catastrophic. I'm guessing this issue is also triggered by cold weather too and where I live (Washington, DC) it's only cold a few months a year these days.