1998 Ford Explorer P1131

The means the 0/2 sensor is reporting a lean mixture. That could be anything from a vacuum leak, to low fuel pressure, to a clogged injector to a bad 0/2 sensor.

Ok thanks, well the code disappeared after putting gas and driving it.

It didn't disappear unless somebody cleared it or disconnected the battery.

I don't know what to tell you haha, I am the only one that drives it and I didn't clear the code. Drove like 30 min highway from work made like 5 min stop at home and came back to no check engine light.

Just because the light went out doesn't mean the code is gone. It's still in memory and likely will appear again.

Checked for codes and there is none. I will keep in mind the things you listed. Do you think that code came up because of low gas? Haven't changed my O2s in a long time anyway to tell if good or not? What type of brand I should look at? Thanks.

Motorcraft or Denso are the best.

Ok, anyway to tell if they are bad? Looked at my Fuel Trims and they are stuck at -100 and never move. Been like that for a long time. Is that a sign they are bad?

The fuel trims are determined by the 0/2 readings so that's not surprising. You can monitor the 0/2 voltage with either a DVOM or live data on a scanner. Below .500v signals lean. Above that indicates rich. It should constantly cross between rich and lean. If it's reading lean, try to artificially enrich it with some carb cleaner or Propane and see if the 0/2 responds by going full rich.

Which O2 sensors do I replace?

The upstream sensors are the only ones that effect vehicle performance.

So got the code p0133, guess that can confirm need new o2. How many do I need to buy?

You haven't confirmed anything but changing the 0/2s for maintenance is always a good thing to do. They lose sensitivity over time.

Oh ok, thought when o2 sensors are slow in response it is the beginning stage of failure. Any idea of how many up stream sensor I have to replace? Thanks

It has 2