Did you clear the codes on the previous iteration (thinking it could just be a persistent message)?

I couldn't find any real info on what's inside that thermostat, but did see one reference to some sort of heating element in the thing that accelerates the opening of the thermostat under certain conditions (sounds like something BMW would over-engineer...). The poster claimed that a healthy heater would measure around 16 ohms, and that if the DME (?) didn't see the heating element it would throw the P0128 code. Take that all with a grain of salt, but it sounds reasonably plausible (for an E60-series car, not a "normal car"). ;-)

Yes, I cleared the code each time. There are already two thermostats, one from BMW so I am fairly confident the heating element is not an issue. Although, the connections to the heating elements could be bad at the ECU. Any idea what pins those are, and where the ECU is?

How would the ECU measure the resistance at the heating element when one pin is tied to 12V from a solenoid?

I wasn't able to find a pin-out of the thermostat, but you should be able to "find" the heater element with an ohmmeter. The reference I did see suggested it was about 16 ohms, which sounds reasonable (that would allow just under an amp to flow assuming full battery voltage, which would translate into about 10 watts of heat, which I suppose makes sense). There should be some combination of two pins that read around 16 ohms on that connector (don't recall how difficult it is to GET to that connector though)...

Ok, I changed the engine coolant temp sensor with a BMW one and no more errors after 1,000 miles.

I'm surprised the Behr didn't work. May have received a bad on. Thanks for the update.

When you changed your heater control valve last year, did you replace the 3 hoses that come out of the firewall (that connect to the heater core under the dash) that also connect to the heater control valve?

Do those little hoses have a mean time between failure before they rip open and leak coolant?

I did not change the heater control valve but if I had, I would have changed those hoses....

I worked on a lot of cars and when it comes to critical engine sensors, I have now learned to ALWAYS pay for the factory ones. Knock sensors, cam sensors, TDC sensors, throttle position sensors, now temp sensors, and also O2 sensors. I once had to do the knock sensors 4 times..,.,,.

My #1 rule is if it makes the car run always get OE/OEM especially on foreign cars. I like to do the job once. I've seen too many horror stories with owners taking the engine almost completely apart to throw in an aftermarket and it not work. Following that rule hasn't failed me in over 15 years of me working on countless foreign cars. Most of the time that extra part is worth it in the long run.

For those not intimately familiar with how this works, OE/OEM doesn't necessarily mean it has a BMW logo, but that it's built by the same manufacturer who builds the part for BMW. So you can buy a Pierburg and get the original pump for a few hundred bucks ($310 on rockauto for example), or you can go to the dealership and order one there, and get the same part, but pay a whole lot more (MSRP $655 - discount online around $500).

Of course, if there is ANY doubt - especially for those hard to reach / change parts, it's worth biting the bullet and helping the dealership owner make their next mortgage payment. ;-)