I've read that K&N filters mess with the MAF sensor and you'll get a CEL, dont know if it's just certain vehicles or what
I've heard that regardless of what vehicle you might install a K&N filter in, you have to be very careful when you do maintenance on the filter. Maintenance includes backflushing after soaking it with a cleaner, and spraying it with an oil product. If you apply too much oil, it can be drawn into the intake system and coat the sensor. Once the sensor is coated, it will give abnormal readings and may set the CEL.
I had read on one of the Ford forums that Ford service departments were told to check for the use of a K&N filter and if found, to determine if the CEL was a result of that filter oil and deny any warranty work involving that system.