Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum - View Single Post - Replacing air box with K&N air filter. HELP
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Old 08-10-2009, 02:55 PM   #11
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> Could the 'lifetime' engine air filter be the problem at 40,000 miles?

Check the sensor which will tell you if the filter is restricting. If it's in the green, no problem.
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