I have 2002 Mazda Millenia 2.5
I got a check engine light the other day, it was code P0400. Then, when I looked under the hood I noticed this big crack on the hose that connects to the air filter box (see picture attached).
Is that crack the reason why I got that code. Also what is the name and part number for that, and where can if find it cheap where it won’t cost me an arm and a leg.

Thank you and you if have any web links to that part please send it.

 

That's the air duct. When it fails, it lets bypass air into the engine downstream of the MAF which is located on the rear of the air filter box. This sneaked air is not metered, so the fuel dose is determined by what the MAF saw going thru the filter, but the sneak air gets into the picture resulting in a lean mixture. A "lean burn" is a hot burn resulting in excessively high NO2 emissions; and it is this that is picked up by the post cat. O2 sensor, resulting in the CEL being turned on (the same effect is obtained if you have a leak in the exhaust manifold or plumbing upstream of the rear O2 sensor).
In your case, you can improvise a splint and duct tape solution, or cut the duct and fit a neoprene cuff, etc. or buy a new one. Dealer's price is in the $40 range, and you can sometimes pick one up on E-Bay.

 

ummm. 0400 is egr flow malfunction. has nothing to do with oxygen sensor. i have never worked on a 2.5, but egr is fairly common. i would first remove the egr valve, and clean it(carb cleaner), and the port(s) all the way to the throttle body. put a vacuum on it and make sure the valve is moving. most of the time this fixes it. you do have to get a new intake boot, too, but this is not throwing the 0400 code

 

The "brane thing" thinks there's an EGR problem because it's sucking huge air back through the exhaust gas recycling loop. It's almost a cert that there will be at least 0171 (lean mixture), 030x (misfire), and possibly 0104 (MAF or intake air leak) codes buried in there too.
Stripping out the EGR and "degrunging" the ports and stem is a bummer of a job if it turns out that fixing that massive split in the air duct takes care of the lot. That's why when you get an OBD read I recommend getting a complete scan with a hard copy printout because a lot of mechanics either use mickey mouse scanners or don't know how to properly use a good one in step-by-step mode.
I point I forgot to mention is that the code(s) will have to be cleared to reset the CEL; this is easily done by leaving the courtesy light on (discharge path for the ECM memory to dump the latched codes) and then disconnect the battery negative connection for 30 seconds or so.
This will necessitate resetting the clock, tuner presets and if the radio is a later model, re-entering the radio code.

 

So you are saying that something else besides the big crack in the air duct made the 0400 code come on?

 

So if this is what was really causing the CEL to come on and fix the crack by either splinting it or replacing the whole thing will the CEL go away on its own eventually or do I have to reset it by other means?

 

Usually for CEL, once the problem is fixed, the light goes out right away but the codes which caused it are kept in memory for "3 drive cycles" , a Mazda term I have not been able to decipher, but which means perhaps a week or so of average driving.