Q: P0171 after oil change

To answer your question as to the cost, it depends on exactly what is wrong. As described below, there are many underlying malfunctions that can cause diagnostic trouble code P0171. Typically, a diagnostic is performed, the exact cause is pinpointed, and the results (including the cost of the repair) are explained to the customer to any desired degree of detail. The causes of lean operation, such as if you have diagnostic trouble code P0171 are easiest to pinpoint by looking at the secondary firing patterns on all cylinders. That will confirm if the too lean condition, that has caused a code to set, affects all or just a few cylinders, and in which bank of the engine (in your case, with a 4 cylinder you have only one bank) . If not all cylinders are affected, a mechanic would 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 - lean operation and the responding certified mechanic will get this diagnosed and resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.