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Q: CEL light P171 p174 codes fuel trim always single digits

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weather idling or driving .. long and short term trim never into double digits

My car has 75000 miles.
My car has an automatic transmission.

If you’re lean on both exhaust banks, typically there would be a vacuum leak causing the issue. Even a small vacuum leak can cause fluctuations in fuel trims and BMWs are a little more sensitive to point out engine operation problems. The DISA valve is a common spot for vacuum leaks and drivability concerns if the valve does not function properly. Consider hiring an experienced technician like one from YourMechanic who can come out and take a closer look at your check engine light troubles and offer a more personal diagnosis.

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