EGR Flow Malfunction, and sometimes a Cylinder 4 Misfire.

Question edited for clarity and the typos - Nissian, cyclinder, vam, and PO 304 is P-zero not P-letter Oh..
Question moved to model category

What makes you think it is the Crankshaft Sensor? This is a simple misfire on Cylinder 4. Check plug, leads and coil. Run some Injector cleaner.

The P0304 code stands for "Cylinder 4 Misfire Detected", which means your vehicle's cylinder 4 has misfired enough times for your powertrain control module (PCM) to notice. A worn spark plug, a faulty fuel injector, and a leaking valve are some possible causes of the P0304 code.
Jul 28, 2023

P0304 Code: Cylinder 4 Misfire Detected - CarParts.com

https://www.justanswer.com/nissan/58ol6-2004-nissan-quest-5spd-where-crankshaft-sensor-located-diy.html

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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

The first fault code do you mean p1296 if so that is a cooling system malfunction you will need to check the temperature sensor.

The fault code p0411 is a Secondary Air Injection System Incorrect Flow Detected

And the p0304 fault code is a misfire detected on cylinder 4 possibly a coil pack maybe a plug

code p0400 refers to exhaust gas recirculation(EGR) system---flow malfunction.===causes---hose leak/blockage--basic setting not carried out (if applicable)---wiring---EGR valve ---EGR solenoid---ECM
check out the valve and solenoid.

P0304 Cylinder #4 - Misfire Detected
P0483 Cooling Fan Rationality Check Malfunction

You'll have to get someone to check the spark and injector on cylinders 3 and 4.
Code 303 is for loss of power on cylinder 3 and code 304 is the same for cylinder 4. The most common causes are lack of spark or fuel.
Once the problems are fixed the codes will need to be erased with a scanner or code reader.

P0400 - Exhaust Gas Recirculation Flow Malfunction
The exhaust gas recirculation (EGR) valve is vacuum controlled valve that regulates the amount of exhaust gases that re-enter the cylinders. The powertrain control module (PCM) determines how much based on engine load, temp, and other conditions. If the PCM detects that the amount of exhaust gases entering the cylinder were insufficient or non-existent, this code is set.

No symptoms will likely be noticed by the driver other than the MIL (malfunction indicator lamp). However, non visible symptoms will be increased combustion temperature and increased Nox emissions. A code P0400 may mean that one or more of the following has happened:
- Plugged EGR passage which restricts the flow of exhaust gasses
- Faulty EGR solenoid
- Faulty EGR solenoid wiring/harness
- Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
- Faulty EGR valve

Check it and keep us updated.

My advise is to look at what moistcabbage had to say about your problem. Then I will add a little advise:
Put the P0171 on hold until you have a verified fix for the other 2 codes. Here's why:
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A misfire causes raw fuel and air to be shoved down the exhaust as an unburned mixture (After all. that's what a misfire is.) This can cause the free oxygen level in the exhaust to rise and cause the lean code (P0171) From the EGR code that you have, I can tell that you have the 2.0L engine in your car. (This is not a valid code for the 2.5L) In this case, moistcabbage is sort-of correct. Thiscode actually indicates that the EGR valve does not move when the computer commands the EGR to open. (Which SHOULD also be producing a P0400 code, which is EGR no or low flow.) If P0400 is not present, then chances are that this is caused by the EGR sensor "lying" to the computer and telling it that the EGR is not opening when it really is. However, it also may be telling the truth and the computer just hasn't picked up on the low flow issue yet. Anyway, if the EGR is not moving, it could be caused by a vacuum leak, which will also cause your P0171code.
The logical order of this would be to check for obvious causes of misfire like the spark plug, wire, coil, and fuel injector and correct as necessary. Also check the EGR system out to find out what the cause of the EGR not moving code is. (And repar as necessary) Then clear the codes and drive the car to see if the P0171 code returns. If P0171 returns and the other codes stay gone, then you will have to check into oter possible causes of the lean code. Otherwise, I would not waste any time checking into a lean condition at all before the other two codes are confirmed fixed.

There is a tube that connects to the EGR valve that gets blocked up-take it off and clean it out

The OBD II code for P0400 is Exhaust Gas Recirculation Flow Malfunction.
I'm sure the only thing you have left is you may have to replace the EGR valve or check for any leak from the vaccum lines going to to EGR valve.