My daughters "check engine light" came on yesterday. Took it to Auto Zone for them to code read and they said it was an EGR valve malfunction. He also said it wouldn't affect the way the car runs, that it was mainly for emissions control. He reset the light and it is out for now. Should I replace the valve or let it go? Also, how tough a job is it to change? The car is a 97 Sentra with the 1.6 engine and 113,000 miles> Thanx.