Igot a check engine light in my 2002 Mazda Millenia 2.5. I took it to Autozone to check the codes and the it showed two: P0400 and P40421.
Can anyone tell me what these codes mean and how I can fix them without spending too much money. If you have pictures or diagrams to help explain the problem thatwillbe better.

 

well, the first one is egr. the second one is no such code on earth. i would suggest you clean the egr valve, all the way to and including the throttle body. the second one, i have no idea.

 

I think the second one was actually P0421

 

i dont think a 2.5 HAS a p0421. a 2.3 does(right bank catalyst below efficiency). at least, not in my 99 manual. however, there IS a p0404-exhaust gas recirculation circuit range/performance. which would make sense since it is throwing a 0400-exhaust gas recirculation flow malfunction.