The MAF has a heated film. There is never any damage you can see but eventually enough contaminents land on the film so the sensor can't give an accurate reading of air flow (starting at high airflow and slowly working down the range usually). The engine light comes on because the O2 sensor are detecting that they need to trim the fuel mixtures outside the normal range (because the mixtures are getting leaner due to MAF fault).

It's still giving a measurement, but the measurement is slowly getting inaccurate......the o2 sensors can pick this up, the ecu can't detect without another sensor giving a conflicting reading.

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32