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Q: CHECK ENGINE LIGHT CODE0171 on 2008 Toyota Corolla

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Has a check engine light on with a code P0171 indicating the fuel trims are too high and the engine is lean.
1. Monitor the fuel trim readings at idle and at 55 MPH driving down the road. If the numbers are low at idle, but climb up in to the +20 range on a part throttle cruise, the mass air flow sensor may be faulty. If the mass air flow sensor is faulty, the calculated load reading typically is low during a wide-open throttle acceleration. The reading should go to 85% or higher. When the mass air flow sensor is faulty it may also have low grams per second readings. The grams per second at idle should be about 1.8 - 2.2 and at 2500 RPM and no load ,the readings should be about 7.2.

3. Monitor both the front and rear oxygen sensor signals. The oxygen sensor should not stay high or low voltage for extended periods of time. If the oxygen sensor shows low voltage and the fuel trim readings climb, check the performance of the oxygen sensor. Some engines that are burning oils have had skewed oxygen sensor readings and odd fuel trims readings.

4. If the fuel trims are high at idle and in the +20 range and drop down below +10 at cruising speeds, there may be a vacuum leak. Check the vehicle for a vacuum leak by spraying carburetor spray around the intake manifold while watching the fuel trim readings. If the readings change in a particular area, there may be a vacuum leak.
Tech Tips:
If fuel trim readings are high at idle and go down on a free rev, this is a vacuum leak. If the fuel trim readings are high at idle and at cruise, suspect fuel injectors.

Bottom line, Wetry is correct. Take it to a professional. This problem is beyond your ability.
Unless you have money to waste.
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