Code p0400 keeps coming up after changing the egr valve and sol.

The fault is somewhere around the EGR valve - probably the EGR valve or the throttle position sensor.

Before replacing any sensors try removing and cleaning the EGR.

Here's a Youtube link that may be of help to you:

1999 maxima egr Google Search

This link explains about the EGR:
P0400 Exhaust Gas Recirculation Flow Malfunction

CEL code P0400 EGR Flow Malfunction

Here's a link Garry. P0400 Exhaust Gas Recirculation Flow Malfunction

EGR valve malfunction. The passages beneath the egr valve may be plugged with carbon deposits, restricting or blocking exhaust flow back into the intake. Or the computer control of the vacuum supply to the valve may be compromised. Take the valve off and clean the bottom and the passages, use a new gasket when remounting it. Check operation, valve should raise as warmed engine raises rpm's off idle. Put a finger lightly on the bottom of valve to feel it raise as the throttle is goosed. The valve is closed at idle, but as engine speed increases, valve is raised to allow some exhaust gasses back into the intake.

P0400 (EGR flow malfunction), P1333 Make sure about this code its a chevy code so you will need to recheck,If anything the EGR is stuck open.Or you are dealing with a vacuum leak causing rough idle

good day Sir, try to use a radiator sealant keep checking it, keep filling it with water, and replace it asap and give the sealant a miss.

Code P1491 is not the PVC Valve.It is the Vacuum Cut Valve Bypass Valve.
Which is located above and away from the egr canister under the driver side rear bumper it is round and have about 3 hoses connected to it. if it is from a check engine light that says look at vacume cut valve check the egr canisters purge valve some times they stick and throw the wrong code

Check fuel pressure it should be 34psi and with vaccum removed from regulator it should be 43psi. Low fuel pressure can contribute to misfires.

there is an EGR vacuum solenoid somwhere that controls on/off vacuum to actuate the valve. i am not to sure of the location, but follow the vacuum line back from the egr until you find a device that looks like an electrical check valve for vacuum. that is a possibilty. but then again i am a ford,chrysler guy and am just giving you an answer based on my understanding of the functuality of a vacuum egr system

Check for vaccume leaks and also you PCV valve. check the hose too. Visually inspect every vaccume hose you can find. Sounds like a leak .

Code P400 is an EGR (exhaust gas recirculation) valve malfunction.

These cars are known for the EGR ports getting clogged.

The valves themselves rarely fail.