My 98 Explorer (115K miles) has the check engine light come on about once a month. Mechanic told me that it was "too Lean" indicator from both banks. He further told me that it occurred at 35MPH during a gradual acceleration. I have a basic code reader and have confirmed for the last two months that each time it came on the codes indicated the 2 banks were showing "too lean". The mechanic has proposed replacing the Mass Air Sensor.
I have seen no noticeable degradation in drivability.
Art McClinton