I found this and this is all it said in another forum on another website:
"PO1131 means lack of upstream heated oxygen sensor (HO2S11)switch, sensor indicates lean ( bank 1 )."
I have no idea what that means but hope it helps a little.
Found this also on another forum:
"PO131 & PO133 are both HO2 Codes.
PO131 is generally associated with a negatiive voltage reading, indicating tha the sensor's electrical connection has been contaminated with water. A coolant/heater hose is probably dripping on it.
PO133 is generally associated with an HO2 sensor that is "below some calibrated window." This basically means that it's not operating at the voltage it should be.
What you have to do now is determine why. PO133 is an upstream sensor so this is mounted on the exhaust manifold. It's not the one behind the cat.
Both of these codes can be the result of water shorting the electrical connection of the HO2 sensor, so this is what I would look for. Make sure that the coolant/heater hose is not dripping on the the HO2 sensor or the wires.
A flashing MIL is caused by an engine misfire. A permanent MIL is cause by a malfuntioning sensor or a sensor that is operating out of range for some reason. When you have a code, you need to start theorizing why.
A couple of things could be happening here. What I believe the situation to be is coolant or water dripping on the HO2 sensor (Passenger side) and causing it to short-out so that PCM thinks the engine is never reaching operating temp. What happens then is that the HO2 tells you PCM to run in open loop/rich mode.
Since this has been happening for some time, the PCM has also adjusted the fuel flow to run rich, permanently. The fuel injector has been pouring so much fuel onto the spark plug that it is either fouled and now misfiring or the injector has been pushed beyond it's limits and now needs to be replaced because it has worn out and can't flow enough fuel.
Check for water on the HO2 sensor and connections.
Many times an HO2 code is caused by a worn out HO2 sensor, but not always. They are there for a reason, so a logical, methodical approach is necessary."
David