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Hi- The SES light came on - but i just replaced the thermostat last year- the gauge needle reads right down the middle. What is the MOST likely cause if it is not the thermostat?Thanks in advance
 

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ROLL TIDE!
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0128 is the thermostat code, it is MOST likely a thermostat. Just because it is new, doesn't mean it can't fail (remember, the Titanic was "unsinkable" and we see how that turned out, pretty sure BMW farmed out parts making to Harland and Wolffe)
 

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I have the same code, but basically during the summer (i live in toronto) the reading is right in the middle. Last week we had a cold front (temps around -2 to 4 degrees Celsius) what i notices is that, if am driving in the city, the temp gauge just fine hits centre, but when am on the highway, if drops to the cold side (sometime even ready way past the quarter mark).

Now, i am wondering, is this exactly what i should see if i have a failed thermostat? And why the highway/ weather dependence.
 

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ROLL TIDE!
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Now, i am wondering, is this exactly what i should see if i have a failed thermostat? And why the highway/ weather dependence.
You are seeing what you SHOULD see. The reason for the variation is simple, when sitting still or going under 30mph the heat will build up more. When on the highway, you get super chilled air going through your radiator and that will magnify the fact that your thermostat isn't ever closing (they fail open mostly unless you are me :eeps:). The reason you don't see this other times is because your gauge has a wide range of temps that show "normal". Normal could be 186 dgerees or 220 degrees, either way it will read middle. With the therm messed up, it is reading really cold and shows you. You need to replace asap because having your car in the blue is actually bad for your car. Needs heat to run at optimum.

And for the record, I have replaced more failed t-stats on these things than I want to remember. Almost all of them did what yours is doing. It told you so with the 0128 code. We don't question the OBD2, we simply bow to its power.
 

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Just because the needle is pointing straight up that doesn't mean the engine temps are actually where they should be. The gauge is so buffered that it points straight up all the way from 167 - 240*F. (75 - 115*C) Use the OBC test mode to view the actual temperature:

1. Hold Trip Reset while turning ignition key to on position
2. OBC should show "Test"
3. Use Trip Reset to select function 19.0 that unlocks all the features
3. Wait for display to show "Off"
4. Depress Trip Reset for 1/4 second and release it
5. With no delay press Trip Reset several times to select one of the following functions

The function you want is 7.0. The temp readout is in C, so a normal temperature will be between mid 90s to low 100s.

Is buying and replacing a t-stat expensive and hard? I've never had to do it before
Piece of cake. Bleeding the system properly is the hardest part. Park the cold car nose-up (on ramps if you only have level ground, but the average driveway has enough slope) turn on the heater full-blast, and start the car. Crack the bleed screw in the upper radiator hose to let air out of the system. Once you get nothing but water coming out, tighten the bleed screw. (Not too much, it can break!)
 
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