99 Nissan Maxima Codes P0325 and P0400

I have a 1999 Nissan Maxima with 79K, automatic transmission and 6 cylinders.

My "service engine soon" light came on and I can't check the codes myself so I took it to "America's Service Station".
Two codes they reported back were
- P0325: knock sensor circuit
- P0400: Insufficient EGR flow.

I saw postings on this site for the second and it is clear to me that definitely should be done. And i have been told the first is important for emissions. But I'm not capable of doing this work myself and I am new to the area so I just don't have a mechanic that I trust.

The problem is that i am getting two completely different quotes JUST for the P0400. One for $180 (service station) and $600 from the Nissan dealer.

How can they be so different? I realize that there is a mark up with the dealer, but that much?

They both do agree that repairing the first is easier to do at the same time because of access to the area where the work needs to be done.

I love this car and want to keep it for as long as possible, I know it will go another 100K... so I want to do the best thing for the car. If it means spending more money, so it goes.

I appreciate any advice!!!
Thanks

compare warranty's and reputation of the chain store versus dealers.
My own opinion will be a lttle biassed as I'm a tech at a dealer but there are good and bad mechanics at both. I will say this, sometimes expertise is worth the money!

anniep,
Has your P0400 problem solved after you fixed it? Could you tell me which part did they change. Thanks

have to make sure the 180$ quote will fix it. The ports are probably stopped up. big job more than 180$ worth.