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Q: I am getting a P0171 code. It runs smooth and the fuel trim is good at idle. Under load the trim goes high.

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The fuel trim increases the more the accellerator is pushed. It runs smooth and does not show any issues other than the CEL with code P0171. What should be checked or replaced? If it was a vacuum leak I would expect the trim to be lean but the trim is 0 at idle. It starts and runs fine do it does not look like a fuel issue.

My car has 106000 miles.
My car has an automatic transmission.

If your fuel trims are ok at idle,but seem to climb when a load is applied to the engine the problem may be in the fuel delivery or air metering. A clogged fuel filter may disrupt the flow up fuel and only allow enough to support idle, but not driving. A faulted air flow meter will offer the computer false reading about how much air is being drawn into the engine and once the exhaust gasses go by the oxygen sensors, the computer sees the difference and makes changes. Consider hiring an experienced technician with a professional scan tool like one form YourMechanic who can come out and take a closer look at the computer live data to offer a more personal diagnosis.

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