From here:
http://www.civicvx.com/lean_burn_info.html
"LAF Sensor Contamination: LAF sensors do not wear out, they typically fail due to contamination. The material that the sensing tip is made of is porous and many types of material will clog the pores or the LAF sensor. As the sensor becomes contaminated it slowly gets slower and slower in its response to changes in the oxygen content of the exhaust. Eventually the sensor may totally fail."