gas mileage

Your code "P1131 - Lack of upstream Heated Oxygen Sensor switch.Sensor indicates lean,bank 1".

What it means.Your O2 is running extra rich due to an overly lean fuel detection.It's one of two things that could cause this. 1.Your O2 is faulty (most probable cause) or 2.You have one or more stuck or partially working injectors on drivers side.I would replace the sensor.Yo can pick one up at any auto parts store.You may also order one if you can find it cheaper.Bosch or Motorcraft are the only brands you want to use.Anything else cant be trusted.Yeah I know they're not cheap.I had to replace all 4 of mine and spent $150.Should be only a one time investment.

Does your car stumble or misfire when the RPM's start getting up near 2k - 3k ? Is it running fine now?

They're not cheap.New,they can go as little as $45 up to $100.Depends on where you get it and what brand name.If your injector is bad,your best off going with a re-man. injector which may only cost you about $25 at rockauto.com.I know your trying to save $$,but if you want guarenteed quality then go with a new one.