My wife has been driving our OBS, with N/A EJ253 engine, and told me the Check Engine light has been coming on and off intermittently. I pulled a P0131 code tonight "O2 sensor circuit low voltage bank 1, sensor 1". 

 

Based on what I've read, this would indicate an issue with the first O2 sensor in the exhaust manifold (or its connection?). If this is cause, can anyone tell where exactly this O2 sensor is located, and the best way to get to it?

 

Also, are there any other causes that would throw this code? 

Depends on the exhaust setup. If it has the dragons den exhaust that wraps around the oil filter the front sensor is usually right at the front on the passenger side.

If it still has the Y style pipe the front sensor will be in the front cat, on top sticking up towards the passenger front axle.

I use a 7/8" box end wrench from above. Kinda tricky to get the box end onto the sensor. Warm up the exhaust first by running the engine for a minute or so. Wear gloves to avoid burns. I usually have to smack the end of the wrench with a hammer to get the sensor to crack loose.

 

Another option is to drop the exhaust to give more room to work, but then you need new gaskets for the head flanges.

Actually I looked down from the top and I do see a sensor on the front side of the cat- so is the is the fron O2 sensor then? I also bought a 3 piece 22MM/ 7/8" O2 sensor socket set on Ebay. I figure one of the three sockets/ wrenches shoudl get it off. 

 

Biggest concern I have is that the things will be rusted in place (this is a northern car). Shot soem PB blaster on it already and will warm it up before I attempt to remove the O2 sensor tomorrow.

 

Anyone have any other advice?

If you can see it from above its probably the front sensor.

 

Rear sensor is in the back of the rear cat way back near the end of the trans.

I saw a couple of Youtube videos a guys with WRX's that had it one the passenger's side where you access it through the wheel well. I'm guessing mine is different since I have a N/A EJ25 vs. a turbo like the WRX.

Yeah the turbo engines have it in an odd spot underneath the turbo. Still easily accessible compared to some I've seen. (Cough... Dodge... Cough)