06 Base with Chrome 3.5 Automatic throwing double "P0135 Bank One"codes. Tried reseting compter several times with my OBD II scanner and it keeps comming back. Its been this way for about 2.5 months with no performance or fuel milage issues until early this week. After starting the vehicle it sputtered and died out when I accelerated but came back within a second after feathering the throttle.
The beauty of OnStar, I was able to call in to them and they ran diagnostics on my SRS, Stabilitrack and OBD system, then informed me of the code and also explained that I could drive the vehicle and it "may or may not affect performance." With some persistence they connected me to the vehicle tech department who seemed to be pretty knowledgeable, (had a good computer in front of him ). Tech isolated the problem to the O2 sensor upline of the Cat. So its off to buy a new O2 sensor, my first google searches found them for around $30-$60. I will try to stick with OEM on this one and will keep you folks updated on the fix.

 

P0135 is the upstream/front O2 sensor (I5 only have 1 bank) and your PCM thinks it is responding too slow "based upon coolant temp".

How's your temps? A T Stat issue can throw that same code if it is sticking or opening late.

 

Funny you bring up temps Doc! My 3 has always run very cool at just above the 1/4 mark. I did a cooling system flush when I first got her a year ago and ran a "cleaner" through there which I learned latter from this forum that by just taking off the lower radiator hose will not flush thoroughly. So there is probably still "cleaner" in there. I also added this "Royal Purple" cooling system additive that someone recommended to prevent the "leak through to transcooler" issue and it was then that I noticed a big drop in operating temp.
Light only came on a few months ago?
The Onstar tech said exactly what your saying that it has to do with "sensor not heating up properly?" Thanks for the feedback.

UPDATE: Found a steal on the Denso OEM O2 sensor from RockAuto.com $41.52 shipped!

 

Got my new O2 from Rockauto.com it was a perfect fit and everything is doing fine. Weirdly though seems like she doesn't have as much pick up when first starting the motor
Here are some pics of the the old sensor where you can see the fouling. Shucks didn't take a pic of the new sensor, but it had more holes on the actual sensor than the stock. Hopefully this will keep it from fouling so soon.

 

Good to know about the thermostat. I'm getting ready to do a radiator fluid change also...

 

Hey HawaiianH3, I;m having the same issues with my H3. Did the new O2 Sensor clear your problems?

 

Had the same code on my 06. As Doc stated, it was the t-stat on mine, cleared the code..