Hi, I'm new to the forum and did some searching but couldn't find the answer I need. Hope you can help!

I have a 2005 Pontiac Montana Extended Base (not the sport model) with a V6 3.4 litre engine.

I have a "check engine light" also known as "service engine soon" light. It'* on steady, not flashing. So I used an OBDII scan tool to pull off the code and it says:
"P0135 Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 1"

So I'm guessing I have to replace the oxygen sensor...fine. But does anyone know where Bank 1 Sensor 1 is located on this model vehicle??? I can't afford to replace all of them so I need to narrow it down...any help would be greatly appreciated!

It would probably be in the same location as the 3800 on the rear exhaust manifold.

Bank1 sensor 1 is located between the ex manifold and the front of the cat( not easy to get to in a van)....bank 1 sensor 2 is located after the cat.....

This is the heated circuit that is causing the problem....have you checked the fuse to the heated circuit? In some cases the wire harness to the O2 sensor, has been known to short out and take out the fuse.....if ok, then it'* usually the sensor......the sensor is probably working, just taking too long to start working without the heater circuit being ok....

Tech II - You are correct, the O2 sensor in "Bank 1 Sensor 1" is between the manifold and the catalytic converter, not after the cat. The fuse was fine. The O2 sensor is hard to get at, so I ended up taking into a shop and they did it. $145 for the part (ripoff) and they also said I needed a new "oil temperature switch" for $35 which is probably another ripoff, and I have no idea what it does or if it was even related to the O2 sensor error code. Labor was 1.5 hours to replace both. Whole thing with tax $338...

Are you sure it wasn't the oil pressure switch? If they replaced the oil pressure switch it was probably leaking which they are prone to prior to failure. A leaking oil pressure switch wouldn't have anything to do with an O2 sensor error unless dripping oil somehow made its way into the O2 sensor electrical connector. At any rate the total bill doesn't seem that far out of line as long as everything is copacetic now.

2004 Bonneville SLE
1971 Chevelle SS

Maybe it was the oil pressure switch, not temp switch, I can't remember...but yes there always was a tiny bit of oil underneath the van so perhaps that explains it. And yes everything is copacetic now, so I guess the indy shop did an acceptable job. I just wish it was the sensor #2 after the cat...because I can see it when I lay under the van and it would have been easy to replace myself, and cheaper because the sensor is about $50 online, instead of $140 at the shop.
Anyway, it'* all good now. Thanks for the advice!