We have a 2001 Infiniti QX4, 38k miles.  About a month ago, the check
engine light came on and we took it in and they said one of the oxygen
sensors needed to be replaced.  Since the warranty had just expired
(less than a month), Infiniti covered the charges.

Now less than a month later, the check engine light came on.  Took it
in and now Bank1 Rear 02 Sensor needs replaced.  My question is should
the previous service found anything wrong with this sensor?  Should I
expect Infiniti to cover the cost of the repairs?  It seem really odd
that 2 oxygen sensors would go out with in a month.  They are quoting
me $325 to fix.

Thanks in advance for any advice.