westsidebalto

Got an "Increased Emissions" warning yesterday in my V8. Had the code checked and it was P0128, which indicates Coolant Temperature Below Thermostat Regulating Temperature.

From what I've read this could be a few things, including low coolant level, open/stuck thermostat, faulty fan, faulty air sensor, etc. Based on other posts to this forum, it seems like the thermostat is the likely culprit.

However, I checked the coolant level and I was bewildered to find the coolant level TOO HIGH. I'd previously had low coolant issues (which went away when I replaced the expansion tank). Could an open thermostat lead to too much coolant in the expansion tank? Should I clean the MAF?

Thanks in advance for anyone who has any tips. I won't be driving the car too much until I figure this out.


Coolant Temperature Below Thermostat Regulating Temperature

Read more at: OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Copyright © OBD-Codes.com
Coolant Temperature Below Thermostat Regulating Temperature

Read more at: OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Copyright © OBD-Codes.com
Coolant Temperature Below Thermostat Regulating Temperature

Read more at: OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Copyright © OBD-Codes.com
Coolant Temperature Below Thermostat Regulating Temperature

Read more at: OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Copyright © OBD-Codes.com
Coolant Temperature Below Thermostat Regulating Temperature

Read more at: OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Copyright © OBD-Codes.com

BimmerFan52

What year is your car? A 545i or 550i?

If the coolant tank overfill is slight it won't affect anything. Simply siphon some coolant out until the proper level is reached. High coolant level will probably not generate the P0128 error.

A common cause for this error is a failed heater coil in the thermostat. When the engine is cold (and the water is not yet heated to open the thermostat via wax melting) and significant power is demanded from the engine, the computer will activate a small heating coil located in the thermostat to accelerate the opening of the thermostat to begin water flow so localized block heating doesn't become excessive.

Upon startup, the ECM checks that the coil is in place by sending a test voltage through the coil to check for continuity. If the circuit is open then an error is generated. If you see the P0128 error code represented by a CEL at or a short time after starting it is likely this coil that has failed. Because the entire circuit pathway is being tested, a broken wire in the harness leading to the thermostat heater coil will generate the same error, but it is less common.

To test, remove the harness connected to the thermostat which will expose two copper blades (contacts). Using a multi-meter with the setting on resistance measurement, place the two probes on the two conductors and read the output. The resistance should measure approximately 16 ohms. If the multimeter reads 1.0 (infinity), this means the circuit is open and the coil is broken.

If the coil is good (resistance about 16 ohms) continue your investigation elsewhere, including a visual check of the harness.

The engine may be safely operated without the coil but the CEL is annoying, and replacement of the thermostat is the only solution.

The only caution to operating without the coil is to be gentle with the throttle until the engine has reached normal operating temperature (a good practice anyway for any engine).

westsidebalto

'04 545i.

It first came on after about 10 minutes of running, which is consistent with what you mentioned.

I ran the multimeter on the thermostat and got about 25 ohms, which was interesting. higher than the 16 ohms, but clearly not broken.

Must be something else. Any other suggestions? (and thank you)

BimmerFan52

Next thing to test is if the thermostat is sticking in the open position.

Start the car with a cold engine and drive the car 4-5 minutes, stop and while the engine is running pop the hood and feel the upper and lower coolant hoses. If one is hot and the other is cold then the thermostat is still closed. If both seem to be the same luke-warm temperature then the thermostat is probably stuck open, preventing the coolant from reaching operating temperature within the time limit set by the computer.

As a second test, drive the car for 20-30 minutes until you are sure the engine is up to operating temperature. Turn the engine off and pull the key. Then restart the engine and see if the CEL is off. If so drive another 15-20 minutes. If the CEL doesn't come back on that is another indication that the CEL is being turned on because of coolant temperature timeout, and the thermostat is stuck open.

westsidebalto

So the lower hose was cool after 5 minutes or so of driving. I let it sit for a half hour in the sun, then ran it another 10 minutes and both the lower and upper hoses were hot.

Unfortunately when I started it the second time, I also got a Passenger Restraint Warning. Just not my week...

BimmerFan52

If one hose is cold and then the both or hot that suggests that the thermostat is closed when you start out and then opens properly.

However, when you receive two or more warnings at random you should look at your battery. It should measure 12.3-12.5V just before starting, 14.2 to 14.5V after starting and after settling in should range between 13.5-14.5V.

The BMW electrical system is a real hog and needs huge amounts of current at the correct voltage to operate all of the computers. When voltage sags the main computer will begin to shed load, turning off systems and it is possible this is your problem.

It can also react weirdly if your voltage regulator is going and it is pumping too high a voltage (I replaced my alternator last year when it was pumping 17V on startup).

The battery maybe not be your problem, but it is easy to check. If you don't have a multimeter any national auto parts store will check your battery, alternator and starter for free. The just hook a hand-held unit to your jump terminal and ground under the hood and you start the car.

westsidebalto

Just had the battery replaced about two months ago. I have a cheapo multimeter that can't do 12v car batteries.

Assuming it's not the battery, I was looking through another post you had about the possible problems:

• Low engine coolant level (Not this, I actually need to take some out, which is odd since I had to top off over the winter twice)
• Leaking or stuck open thermostat (Possibly still this?)
• Faulty cooling fan (How would I know it's running too much?)
• Faulty ECT (Checked both of these and both have resistance)
  
• Faulty intake air temperature (IAT) sensor (this is part of the MAF? How do I check this?)

Any chance the water pump is involved?

BimmerFan52

Water pump - Not unless the car has also exhibited overheating.

I brought up the battery because you said you saw a second, different fault. If it is an alternator problem a new battery won't help for long. Regarding your cheap multimeter, it doesn't measure DC voltage? Have an auto store check it for you.

I agree coolant level is not your problem.

You had one hose get hot with the other cold. Generally this means the thermostat is closed, which is what you are looking for. Later you said both hoses were hot, which means the thermostat has opened, which is good. You also mentioned that the heater coil is present so you can discount that. However, the thermostat could still be faulty if it is opening too soon or as you say leaking (not fully closing). After discounting the rest of the possibilities you may want to change the thermostat (not too bad a job on the 545i). It sucks to start throwing parts at a problem, but sometimes you get to that point (in fact many dealers seem to do that as a starting point). If your water pump and thermostat are original, you are way overdue anyway to do an entire coolant system upgrade (pump, thermostat, hoses, belts and vent tube). You said you recently changed the coolant tank. Overheating can be far more dangerous than an emissions CEL.

Faulty cooling fan - Does the fan come on before you get the CEL? If not the fan is not your problem.

ECT Sensors - Here it is not enough to just measure that a resistance exists. If you are to learn anything measuring yourself, you need the manufacturer's chart of voltage to temperature. The ECU sends a static reference voltage through the sensor (usually 5V) and then measures the return voltage, and from the voltage drop (which changes with temperature) calculates the temperature. To test one you need to remove it from the car, put it in water with an accurate thermometer and send a steady 5 volts to it while increasing the temperature of the water and taking temperature and voltage measurements that you then compare to the chart (yeah, not a trivial exercise).

IAT Sensor - This affects the air fuel mix and I believe is part of the MAF on the 545i.

My recommendation is to check the battery so that can be discounted that first. You can also get some MAF cleaner and lookup instructions on this site for safely cleaning it. Be careful handling it as they are expensive.

Then start looking at your temperatures using the secret menu and see what the temperature is when the CEL kicks on (I think you said it takes about 10 minutes).

Instructive also is the temperature when cold starting. If the temperature is way off the ambient that will be a strong clue that the ECT sensor is bad.

Instructions for secret menu attached if you haven't visited it before.

westsidebalto

CEL came on after 10 minutes the first time...now it's on all the time and hasn't shut off.

I'm going to look at the temperatures and then the battery later today.

And thank you again...

westsidebalto

Well the car sat for 12 hours and the ambient air temp is 28 and the engine is 36. I'll have to check that again later after letting is sit longer. Seems like it might be ECTs