2003 Grand Am 3.4L v6, with EDC P0131 and MPG around 18

Normal MPG on my car is between 23 and 29. The last tank it was about 18 mpg, ALL with 89 octane gas.

Engine is well maintained, but it just received a new ATF fluid change after some leaking.

The main problem that led me here is that the 'service engine soon' light tripped about the 20th of September on a 13 day road trip. We have put a total of about 4000 miles on it since it tripped, as it was running smooth and was getting high 20s in MPG on the trip [the average for the trip was 29.5 mpg].

I just purchased the Actron code reader and read that code listed above [P0131: O2 Sensor Circuit low volts bank 1 sensor 1], then cleared it.

What should I be doing, and where, to fix this? I don't know which O2 sensor is Bank 1 Sensor 1 [before or after the cat?].

I was reading that the engine automatically begins to run a bit rich when the service engine light is on. How likely is it that my cat is getting plugged? The cat sensor was not tripped, or there isn't one for my car: I can't remember which.

Any tips on how to fix it now that I know what the code says?

Thanks.

what the heck is a cat sensor? you'll find a pretty good idea about which sensor and how to fix it by looking in the forums!