Well, ever since I put the new o2 sensor in my car back in August or September I've been getting intermittent 'Check Engine' lights.
Never would come on when I had access to a scanner, until yesterday in Grand Forks, ND. I had just pulled into the Advanced Auto parking lot when the light came on so I left the car running and had them bring out their Actron CP9140.
They pulled 3 codes:

P0131
Oxygen Sensor
Signal Is Too Low

P0171
Fuel Trim
Sensor Signal Lean

P0629
Pass Key
Frequency Invalid

The last code is understandable -the VAT'* system wires were snipped as it wouldn't allow me to start the car at the last National Bonneville Club Meet in Rockford, IL.

The others really just tick me off.
I drove home from Grand Forks (210 miles) at 75-80 mph, all highway, on a little over half tank.
The next half tank consisted of 25% in town, and the other 75% highway, and it got me a grand total of 80 miles.
In other words, in town, at low speeds the car is sucking gas up like it'* nobodies business. I did give the car about 4 full throttle runs up to 30-35 mph, but that shouldn't take up more than 1/10 of a gallon for each of those.
So what the heck is going on -the CE light comes on primarily when the car is idling when warm (190-205*). I can get the light off if I go full throttle for a mere second, restart the engine at speed, put back in gear and coast for 3-5 seconds (hitting the throttle during this time keeps the CE light on).

Anyhow, is there a warranty on these o2 sensors when bought from a GM dealer? I know Bosch and a couple other brands have 1 YR warranty'*. This AC/Delco o2 sensor has no more than 10,000 miles on it and less than 5 months.
In other words this was a spendy 02 sensor, which has been causing problems all along, starting the day I put it in the car.
I am ordering a new one this week, but before I do is there a warranty on it so I don't throw it away -$81 2-3 times a year is too much to be spending on o2 sensors for a Pontiac Bonneville if you ask me.
If the next one does the same thing I am going switch over to some other brand with a warranty.

When my O2 sensor went I would also get the SES light once the car was warm. Replaced it and never saw it again. I got my O2 Sensor on ebay. AC Delco for $19.99, and it'* still just fine.

hmmmm just a thought, are you running e-85 fuel by any chance? every once in a while when gas prices spike i fill up a tank of that stuff considering its 1.69/gal vs like 2.something for regular gas

when i use e85 the car gets worse gas mileage and when it idles for a while while warm the ses light comes on but like 10 secs later after you press the gas it goes back off.

just a hunch thats all.

e-85? Not me -went to a gas station in Grand Forks where I had gotten 102 unleaded last year -the pump still has the 102 unleaded race fuel sticker , but just above that there is a freshly place E-85 sticker so I confirmed it with the attendant before I pumped.
Otherwise -nothing but 92+ octane goes in my car unless the best I can at that point is 91.
My car is SC, and lightly modded to where it should really have 93-94 octane.

Yeah, E85 can cause havoc on fuel systems not designed for it, like degrading seals, as well as requiring altered timing. I don't suggest running it.

BTW does anyone know if the AC/Delco o2 sensors purchased from GM dealers are warrantied at all?

the PO131 is setting off the PO171

Dealer -Needed one bad because the old one was melted, and I was headed to the track the next day. Couldn't wait for one from GMPD.

Do you have any exhaust leaks around the sensor, rear manifold, cross over pipe etc?
Feel around with gloves on, you might feel a pulsating air leak?