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Old 08-13-2012, 04:56 PM   #1
Dre325
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Join Date: Oct 2002
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My Ride: 2001 325i
P0128 Code, AC not working. Need advice!

Hi all,
My 2001 325i (~95,000 miles) is throwing the P0128 code which I understand is typically a bad thermostat. I am initially thinking of replacing the thermostat and expansion tank and water pump as well as doing a full coolant flush. My belts are new and coolant is full. I don't want to have to replace hoses as they sound like the hard part, but maybe someone can convince me I need to.

Here's the problem though: I recently turned my AC on for the first time in probably months and it made a strange noise which has since gone away, but it doesn't blow any cool air whatsoever. Reading other threads, it sounds like the final stage resistor (which I've changed twice) and coolant temp sensor could also be possible culprits to the P0128 and AC issue. I'm wondering if the two issues are related...

Can anyone provide tips to help me root cause this? I have a code reader so can read sensor temps, etc, but don't know where to start. Replacing the thermostat looks easy, but getting my AC to work is very important so need to get this root-caused and resolved ASAP.

Last edited by Dre325; 08-13-2012 at 05:14 PM.
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Old 08-14-2012, 04:14 PM   #2
Dre325
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Can anyone help or point me in the right direction with these issues??
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Old 08-14-2012, 04:24 PM   #3
trj
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P0128 is generally a stuck open thermostat, or the sensor as you already know.

If the temp needle doesnt get to 12 o'clock and stays a little on the left of the 12 o'clock position even after the car is fully warmed up(about 5 minutes of driving) that means the thermostat is stuck open and the coolant is going through the pipes all the time.

Since you are already doing the pump, thermostat and the tank, do the hoses as well. Hoses are cheap and the earliest point of failure.
Upper hose is super easy, the lower one is a little tricky. It houses the sensor too. It would be a good idea to get that sensor replaced as well as you will already be there.

As for AC whacking out, I dont think it is related to the P0128 code. When the thermostat is stuck open, the heater does take time to blow full heat though.
What does the OBC read about the ambient temperature? Most of the times its the temp sensor(in the driver's side fender well) that craps out. If it is reading normal, then may be FSR or something else.
I am not very familiar with the AC system, but P0128 definitely. I had the same code and changed the thermostat and the hoses.
Doing coolant flush and water pump pulley soon.
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Old 08-17-2012, 01:13 AM   #4
Dre325
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Thanks for the response. I checked the OBD temps today on my commute home from work. I checked the local temp on my phone first and it was 89 degrees F outside. On the hidden OBD 7.1 (ambient temp), I got a reading of 275 with the car off. This varied up to 420 on my 30 minute drive home. I am assuming this is supposed to be in degrees C?? Can somebody confirm that I definitely have a bad ambient temp sensor in the driver's side wheel well?? Sure looks like it, unless I am interpreting the readings wrong. Hopefully this is what is causing my AC to not work.

On the 7.0 coolant temp readings, I had a reading of 33C when first starting my car (or 91F which seems right as it was about 90 outside). It climbed to about 88C (190F) after 10 minutes sitting in stop and go traffic. It then dropped down to 81C (178F) when I finally got up to about 50mph on the freeway. Seems like I need to replace the thermostat, correct?
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Old 08-17-2012, 06:29 AM   #5
shanneba
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The ambient temp reading in the OBD menu is in degrees C, xx.x
your temp was 27.5C (81.5F) and it went up to 42.0C (107.6F), that should not be the cause of your A/C not working.

The FSR controls the fan speed, if your fan isn't working properly on the auto setting or manually it may be the FSR.

If your "code reader" will connect to the Heating and A/C system you can read the high side A/C pressure as well as the heating and A/C temperature settings and sensors.
Is the compressor engaging? Is the belt still on the compressor?
Check the fuses for the A/C, there are several including a small one for the hot water valve.
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Old 08-17-2012, 10:45 AM   #6
trj
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Quote:
Originally Posted by Dre325 View Post
Thanks for the response. I checked the OBD temps today on my commute home from work. I checked the local temp on my phone first and it was 89 degrees F outside. On the hidden OBD 7.1 (ambient temp), I got a reading of 275 with the car off. This varied up to 420 on my 30 minute drive home. I am assuming this is supposed to be in degrees C?? Can somebody confirm that I definitely have a bad ambient temp sensor in the driver's side wheel well?? Sure looks like it, unless I am interpreting the readings wrong. Hopefully this is what is causing my AC to not work.

On the 7.0 coolant temp readings, I had a reading of 33C when first starting my car (or 91F which seems right as it was about 90 outside). It climbed to about 88C (190F) after 10 minutes sitting in stop and go traffic. It then dropped down to 81C (178F) when I finally got up to about 50mph on the freeway. Seems like I need to replace the thermostat, correct?
I dont think there is anything for the P0128 to do with AC. Erase the code and see if it returns. For AC, I cant give you much information but I am pretty sure that its not caused by the thermostat. You should get the AC checked by a qualified tech, our AC systems are known not to be messed with, with little knowledge.


Did you check the temp gauge after driving for sometime. Does it stay at the middle or slightly to the left?
If slightly left of the 12 o'clock position, classical case of open thermostat.
Change the thermostat first and see if it helps the AC issue too, I doubt it will but never hurts to try.

The only issue you will face with open thermostat is, the engine will never reach the optimal temperature, hence more fuel will be burnt. Result: a little decrement in MPG.
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Old 08-17-2012, 02:19 PM   #7
Dre325
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Quote:
Originally Posted by shanneba View Post
The ambient temp reading in the OBD menu is in degrees C, xx.x
your temp was 27.5C (81.5F) and it went up to 42.0C (107.6F), that should not be the cause of your A/C not working.

The FSR controls the fan speed, if your fan isn't working properly on the auto setting or manually it may be the FSR.

If your "code reader" will connect to the Heating and A/C system you can read the high side A/C pressure as well as the heating and A/C temperature settings and sensors.
Is the compressor engaging? Is the belt still on the compressor?
Check the fuses for the A/C, there are several including a small one for the hot water valve.
OK, thanks for helping me interpret the ambient temp. It got to 420 (or 42.0C/107F as you say) when sitting stopped in traffic so it does actually seem reasonable that it was that hot under the wheelwell.

Fan works fine so I don't think it's FSR.

I can hook up an OBD code reader to check the pressures and temps you mention. The problem I always have is interpreting what is normal and what would indicate a problem. If you know what these readings should be, please let me know.

Yes, the belt is still on the compressor and it appears to be engaging. I'll have to check the fuses--I don't know how to tell if there is an issue there, but I'm sure I can figure it out.
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Old 08-17-2012, 02:24 PM   #8
Dre325
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Quote:
Originally Posted by trj View Post
I dont think there is anything for the P0128 to do with AC. Erase the code and see if it returns. For AC, I cant give you much information but I am pretty sure that its not caused by the thermostat. You should get the AC checked by a qualified tech, our AC systems are known not to be messed with, with little knowledge.


Did you check the temp gauge after driving for sometime. Does it stay at the middle or slightly to the left?
If slightly left of the 12 o'clock position, classical case of open thermostat.
Change the thermostat first and see if it helps the AC issue too, I doubt it will but never hurts to try.

The only issue you will face with open thermostat is, the engine will never reach the optimal temperature, hence more fuel will be burnt. Result: a little decrement in MPG.
The P0128 code disappeared a few weeks ago and hasn't come back, but it's been warm here so that may be why. What I need feedback on is, based on my coolant temps I mentioned previously, are my coolant temps lower than normal and indicate a thermostat problem? My temp gauge is at 12 o'clock. No issues there and I've read this isn't a good indicator of an issue anyway.

I understand I could get this all checked out at a shop, but I'm trying to avoid that cost if there's something I can do to fix it myself, thus my post here.
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