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P0171 System too Lean (Bank 1)
P1131 HO2S 11 Indicates Lean (Ford, Mazda)
They indicate that the O/2 Sensor is picking up on a lean condition. There can be many causes for the but the most likely one is a bad vacuum leak. With the engine idling, listen closely for a hissing noise and follow it to it's source if you hear one. You can also get a can of carb cleaner and spray it in suspect places. The engine will change pitch if you hit a leak. There are other possibilities too like MAF/MAP sensor, low fuel pressure or even a bad O/2 sensor but I doubt that..