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Q: What could be the cause of code P0171

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The car shows code P0171 (Mixture too Lean), already changed both O2 sensors, but when reading live OBD data, shows 0 Volts on sensor # 1 , could that be the cause of the code?

Or could it be a PCV valve, or a vacuum leak?

My car has 123000 miles.
My car has an automatic transmission.

PO171 occurs when the fuel mixture has gotten so lean that it is out of the range that the ECU can compensate. It can be accompanied by poor idle quality, weak performance and even stalling. While Oxygen sensor number 1 can be the cause, that is usually not the case. More often a P0171 is caused by vacuum leaks like bad vacuum hoses, the air boot connection to the Mass Air Flow sensor, (MAF) or even a bad MAF itself. Check under the hood for bad vacuum hoses or air lines and correct anything that you find. If you don’t find a problem there, you should get a professional to check it out so you don’t keep buying expensive parts that you don’t really need. By contacting Your Mechanic, you can have a technician come to your home or office to check out your VW and let you know what it will take to fix it.

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