DTC Code P1131

[Kraze]

My car will randomly throw this DTC code. It will be there for a while, then it will go away. The car is a '97 with 122k miles on it. O2 sensors have never been replaced as far as I know, but I got the car with 100k miles already on it. Does this definitely indicate that I need to replace the O2's, or could it be something else?

Some extra information, the car is lucky to get 13 mpg in the city, 22-23 is about the max it will get on the highway. And that's with me babying it, trying to get good gas mileage.

Here's the code description: P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1

 

[kustomizingkid]

I'd chunk in O2's just because it could use them anyways... if it comes back I'd look for vacuum leaks...

 

[Tiltedhalo]

A small exhaust leak next to the primary O2 will set that code, even a small leak.

 

[Kraze]

Which side is bank 1?