improving mileage 102
check for codes
troubleshoot code related causes in logical sequence
replace or repair one part
check mileage
repeat the above until problem is resolved

If you prefer you can start with--
Check air filter
check spark plug(s) replace all if checked plugs indicate
Replace fuel filter--easy and inexpensive
Check mileage and if code is still there

There may be a better sequence, It would be good if JCL provides his, but a good way to make a X expensive is to throw parts at it when you have a problem.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas