P0401...

As hotelprisoner said. The manifold sits between the throttle valve and the engine intake. When the engine is off and no air is moving, the pressure will match atmospheric, around 14 psia.

When the engine is running, that will be lower, because the engine takes air from the manifold, and the resulting reduced pressure there is the only reason normal atmospheric pressure will push new air in through the filter and throttle. It seems to hang around 4 psia (or you could think of it as −10 psig) at idle, when the engine isn't revving very high but is sucking against a mostly closed throttle. If you goose the throttle, the pressure will increase (or on a vacuum gauge, the vacuum will decrease, same thing) as the throttle valve opens but the engine isn't yet revving higher and using more air. But of course the engine responds by revving higher, and when it finds a new equilibrium with your new throttle opening, the pressure is probably back near the old 4 psia again.

Generally, the lowest pressures (or highest vacuum-gauge readings) will be when you have been revving the engine and then you release the throttle. The engine at high revs pulling against a nearly-closed throttle might pull the pressure down to 1 psia or less; that gives you the resistance that's useful for engine braking. That only lasts until the engine is back down to idle speed again, and the MAP will be back around 4ish.

But the main thing you'd likely be checking for if you're doing a test involving the MAP sensor would be the operation of the EGR system. In fact, I think that's the main reason the MAP sensor is there. Gen 1 and 2, which did not have EGR, also didn't have MAP sensors.

With EGR, you can be at idle with the normal MAP reading around 4 psia, and start opening the EGR valve some, and watch the MAP reading go up. Unlike the temporary change when you goose the throttle, which only lasts until the engine settles at a higher rpm, the change when you open the EGR valve doesn't increase the idle speed because you're just letting in stuff that doesn't burn. So the engine keeps idling about the same speed but the MAP reading is higher. (At least until you have the EGR opened far enough that you're starting to smother the engine.)

The ECM itself does that test from time to time; that's how it convinces itself the EGR is still working. It only does it at certain moments when the engine is warmed up and you are decelerating with no accelerator pedal input.

 

Well ... hard to say. The code only tells you the car thought it was opening the EGR valve but the MAP reading didn't change as much as expected, and now with this valve you've just put in, the MAP seems to be changing enough. You might look at this recent thread for a way you can use Techstream to see what the actual pressure change was the last time it ran its test. (Four years I've had this car and I still stumble on things in the manual I'd never noticed before.)

Sometimes, once you've replaced a thing and the problem seems resolved, pressing further to pin down just what went wrong is interesting but also real work; it can take some cleverness to even devise an experiment that would answer your questions about it, and then would take more time and additional messing with the car to do the work, analyze the results, and see what conclusions you can draw from it, if any. I really do that sometimes, because I'm weird that way, but even I don't do it every time. You have to decide how curious you are, and how much work that's worth to you.

 

No special information. Standard six-terminal step motor, permanent magnet rotor, very little to go wrong. There was a thread in the past year, regarding Gen 4 engine swaps to Gen 3 v, that included some extensive discussion of the EGR with the circuit diagrams and expected oscilloscope traces, all found in the repair manual.