AWD Disable signal

Just  after changing the air filter,  I
drove my car like 2 miles and parked  it
and entered  Walmart.  I came out and
started  the engine  and the AWD
Disable  light came on.  I drove it
thinking  that the signal  will disappear,  
but it didn't.  I also checked  the tire
pressure  of my tires. The signal is still
on.  I went  to a mechanic  and he told  
me that the differential  oil  is  full,  the
signal  may be caused by a  faulty
sensor. So what do you advise me to
do next?  Keep  driving  or flush  the the
oil and change  the oil with  new oil?