P0400 1999 Nissan Altima Sedan -

Question

p0400, p0171 and p0325. What is the best guess as what needs to be corrected?

Answer 1

P0171 could be caused by an intake leak, dirty mass air flow sensor (try to clean it with mass air flow sensor cleaner) 

https://www.autocodes.com/p0171_infiniti.html

P0400 is set by the EGR, due to the age of this car the egr might be clogged by carbon build up

I would replace it and clean the egr and its piping. Use a wire brush, screwdriver, wd-40 (will loosen the carbon buildup) and brake cleaners(use it last to clean everything off)

p0325 is set by the other codes, so nothing to do there