Any idea what the most common cause is? the egr valve on 99s looks almost inaccessible. Is this a common code/ I get one then it clears for a few months then comes back.
Is this a pattern/common failure in the Max?

 

Common problem. The most likely cause is a clogged EGR port on the plenum and/or a clogged EGR guide tube. You can clean both out yourself if you are reasonably handy - since it involves pulling the plenum off the car. Get a Haynes manual (if you don't have one)...

 

any idea why it would be intermittant? that is get the light it goes away weeks later comes back a month later?

 

The tube is probably partially clogged - so sometimes the flow detected is sufficient and sometimes not. Mine did the same thing. The light popped on, indicated P0400 (0302), I reset the ECU and the light didn't come back on at all. I had to pull the plenum for some other repairs, so cleaned out the EGR port and tube at that time.