Just got my 4Runner (99 SR5-4WD, 81K miles) back from the dealer.
Thought I'd share what I learned from this experence. I took her in
because the check engine light came on. Problem was found to be the
Air/Fuel sensor. Following discussion with the mechanic, believe the
sensor could have been damaged by use of K&N filter. Here'* why,
installed the K&N filter about a year after I purchased the car.
About a month ago, I cleaned/oiled the filter. Soon after, the check
engine light came on. According to the mechanic, the sensor and the
surrounding area were dirty. He thinks the oil residue from the
filter could have caused this by being drawn into the air intake.
This is not the only vehicle I have with a K&N, (97 Chev Silverado,
and my 92 SSEi with an amsoil filter) and this is not the first time
I have cleaned/oiled the filters. Also I am very careful not to spray
too much oil on the filter, as I am aware that the possibility of oil
residue being sucked into the air intake. Moral of the story, don't
know for sure if this was really the cause or not but those like me
who use the K& N, just be aware.