Q: Q: Engine light on with P0171 code

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Good morning. I'm having issues with my truck. (2002 GMC Sierra 1500 SLT 4WD)I bought it from a used car lot and the truck runs well no noticeable issues. Three days after I buy it boom engine light turns on. I'm really upset because the lot I bought it from more then likely reset the codes before I bought it because I had 3 codes pop up 3 days after I got it. What a coincidence, right? Anyway ran a obd II and had codes P0440, P1626, P0171. Went to a shop ran a diagnostic and they told me The PCV valve needed to be replaced and also the evap canister as well because it had water in it. 390$ later I'm driving with no engine light. We'll truck drives fine and no issues I don't smell anything and 1 day after I get it from the shop the Engine light turns on again. It's P0171 (system too lean bank 1). Any suggestions on where to start? MAF sensor is clean and I was thinking about a changing the fuel filter today. How can I pinpoint the exact issue?

My car has 149029 miles.
My car has an automatic transmission.

The cause behind diagnostic trouble code P0171 is easiest to find by looking at the secondary firing patterns on the Bank one cylinders. That will confirm if the too lean condition, that has caused the code to set, affects all cylinders in the bank. If not all cylinders are affected, test for defective individual fuel injectors. However, if all cylinders are affected, you would test, in turn: the fuel pressure regulator, fuel pump pressure, check for a clogged fuel filter, vacuum leak, faulty oxygen sensor(s) and faulty mass air flow sensor. Rarely, the cause will be a faulty powertrain control module. If you want these steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a check engine light diagnostic - code P0171 and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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