Check engine light keeps coming on on my 2006 5.9 2500 , and reads air mixture bla bla. my mechanic is still trying to figure it out. the preveous owner had put an air box and big exaust.

Sensors come in different qualities and pricings. Some cheap ones made in Mexico fail in a few days. Always buy OEM quality sensors.

Your code is incorrect fuel mixture, sensor says running lean so it is dumping extra fuel.

Fuel Trim System Lean Bank 1

The code is thrown when the oxygen sensors detect that there's not enough oxygen in the exhaust system. This is commonly referred to as a "lean" condition.
The vehicle's ECM will add more fuel to the mixture in order to bring the air fuel mixture where it needs to be for optimal combustion. It also throws the code P0171 to let you know that it has done this.


Here are the typical symptoms that something is wrong when you have code P0171.

• Mileage- may or may not suffer depending on how much the air fuel mixture has changed.
• Misfire- If the air fuel mixture has gotten too far from factory specs, this can cause the Sentra to misfire.
• Idle Issues- With the air/fuel mixture altered, the vehicle can idle erratically as the computer compensates to keep the engine running.

https://www.google.com/search?q=Nissan+Sentra+code+p0171

Ideally you need an auto electrician with a pro grade scan tool...

that is a fault and indicates a problem heated oxygen sensor in the exhaust manufold
That is the sensor that reports to the ECM so that the air /fuel mixture can be properly managed
other items affecting mixture are
faulty injectors
leaking injector seals
excessive fuel pressure

Typically, a flashing light is more serious than a solid check-engine light. The flashing light is meant to alert you that you may have a misfire in your engine that will cause catalytic converter damage if you leave the problem unresolved. Replacing a catalytic converter can be costly, so have the engine checked as soon as possible by a certified mechanic.
A flashing check-engine light generally indicates that there is a failure in the emission control system. The emission control system contains an array of sensors that work to reduce the amount of pollution released into the air. An oxygen sensor reads the air and fuel mixture and lights up when it is out of range. The acceptable range is between 50 millivolts and 800 mV. If the check-engine light is flashing, you should visit your mechanic or dealer as soon as possible.

This is the closest I've come to the code you are looking for. I searched all Dodge codes and didn't see any with a "point 2" but I found code P0172. According to EngineCodes.com your fuel mixture is probably too rich. This is what the site had to say:

Possible Causes for this Code:
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Exhaust gas leaks
- Incorrect fuel pressure
- Faulty Mass Air Flow (MAF) sensor
Read more: http://engine-codes.com/p0172_dodge.html#ixzz35waCcFhN

This code is detected when:
- Fuel injection system does not operate properly.
- The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.)
Read more: http://engine-codes.com/p0172_dodge.html#ixzz35waXE700

Possible symptoms:
- Engine Light ON (or Service Engine Soon Warning Light)
- Excessive Fuel Consumption
Read more: http://engine-codes.com/p0172_dodge.html#ixzz35waruoIU


P0172 Dodge Code Description:

With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios.
In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the MIL (2 trip detection logic).
Read more: http://engine-codes.com/p0172_dodge.html#ixzz35wbfFdyj


Hope this helps!

There should be a trouble code or codes stored in the engine computer. A hand held scanner could retrieve them-go to Autozone for a free scan service. The codes indicate to a mechanic where to start looking-whether it is a fuel, or air, or ignition spark delivery problem.
Things that should be checked are the air filter, fuel filter, spark plugs, check fuel pressure, check for air obstructions into the engine (like a dirty throttle body inside-sometimes a cause of stalling), check for vacuum or air leaks that let unmetered air into the engine. There is an ideal ratio of gas to air mixture that the computer is always trying to achieve. When it does, the engine runs fine and gas mileage is optimum. This is why it is important to keep up with regular maintenance and service. Get the codes to see if the computer is doing what it does best, better even than an old mechanic with a good ear.

Check to see if the check engine light is on. Then double-check your vacuum lines. Another thing it could be is the MAF ( Mass Air Flow ) sensor. 90% of the time the Mass Air Flow sensor heated element which meters the air so the computer can add the proper fuel mixture has fallen out of specification. It will cause the Air and fuel mixture to be so off it can cause Stalling, poor acceleration, won't rev over a certain rpm, and running issues. You should have the Air and Fuel mixture readings checked to verify. If found to be bad, do not replace with a After Market one. After market, MAF sensors never seem to work properly. Their heated element doesn't work right, which can cause 02 sensors to flat line and have you chasing your tail trying to figure out what is wrong. So the only way to correct this issue if you do have a bad MAF sensor is to replace it with an OEM.

What's the CEL code? Most mechanics try to avoid EVAP codes, if that's what it is.

Anytime you make a change to any part of the emissions system, its best to disconnect the
battery for a few minutes. If you don't, the computer retains the fault code and it will usually keep
coming back. When all power is lost to the computer, it will do a complete scan when reconnected
and engine started. Try disconnecting the battery for a few minutes then reconnect and see what happens.

No the light is fine its the computer or there is a intake leak somewhere causing the mass air flow sensor to not work correctly, also a cracked hose may be giving a false reading. Get a mechanic to test it with a voltmeter and ohmmeter using a vacuum gauge to make sure it is the mass air flow sensor