My car studders from first to second. No transmission codes . possible MAS issuse both read outs sensors came up lean fuel mixture. Opened the MAS an it was clean but cleaned it anyways. The service engine light is still on. Could I just need to replace the MAS even though its clean? maybe wiring is wrong? or bad? Also said fuel injectors could be bad. Heated oxygen sensor? what could it be? Fuel pump??

code p 0171 refers to system too lean===causes --intake / exhaust leak---AIR system---MAF/VAF sensor fuel pressure ---pump---injectors/s --HO2S sensor
two things stand out
air leak inlet manifold --check for vacuum leaks in vent controls , transmission actuator , brake master cylinder and vacuum hoses
heated oxygen sensor (HO2S) faulty
basically the ECM gets a report from the HO2S and adjusts the air /fuel mixture accordingly so if that unit is faulty or the wiring is faulty then the ECM cannot maintain correct mixture
also the ECM gets reports from the mas air flow ( MAF) or volume air flow) and accordingly also along with the HO2S report adjust the throttle position as well
I would be taking it to an accredited service shop that works on your make of vehicle and discuss the code and the causes with them so that it can be fixed

P0171 - System Too Lean Bank 1 Share
1 ' Add Comment Possible causes
- Intake air leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Faulty Mass Air Flow (MAF) sensor
- Incorrect Positive Crankcase Ventilation (PCV) hose connection
What does this mean? Tech notes
If the P0171 is combined with the P0174 code, it's very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced.
What does this mean? When is the code detected?
- Fuel injection system does not operate properly.
- The amount of mixture ratio compensation is too small. (The mixture ratio is too lean.)
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
- Excessive Fuel Consumption
P0171 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 Malfunction Indicator Light (MIL) (2 trip detection logic).
Read more: http://www.autocodes.com/p0171.html#ixzz3t65yA9xI

Since you have replaced the sensor, my first guess would be low fuel pressure or a vac leak.
Code 171 means the computer can not make the fuel mixture rich enough to make the sensor switch from lean to rich and back.

code p0171 refers to system too lean bank1===causes intake/exhaust leak-AIR system--MAF/VAF sensors---fuel pressure/pump/regulator---injectors---HO2S. To understand the fuel mixture there is a direct ratio between the air and the fuel and it is 14:1 air to fuel . Anything either side is too rich or too lean. manifold leaks/vacuum hose leaks--Mas air flow sensor, Volume air flow sensor all tell the CPU what the ratio is so any unit not performing will throw out the ratio. If the fuel pressure is too low or high the injectors do not squirt properly at the rpm required

Not exactly.
Codes 1151 - 174 - 153 all point to a faulty O2 sensor bank 2 sensor one. That is the sensor ahead of the catalytic converter. All three codes state the sensor is reading the fuel mixture is too lean. It could be lean but you should also have codes for the sensor on bank one.
Code 401 is for a low air flow for the EGR valve which would probably be a different issue. Its possible the EGR valve is faulty and that could affect the fuel mixture but usually for the EGR to cause a lean mixture the EGR would cause a vac leak not fail to open.

one of the cylinder banks is running lean on fuel.you might have to get the injectors cleaned or new ones.there is a lot of things that cause this trouble code for instance a faulty O2 sensor.or a mas air sensor.all these little sensors that tell the engine control module when to advance more fuel.it shouldnt be tha hard to track down if you take it to a repair shop that can scan the trouble codes.

sounds like you have a vacuum leak. It is sucking in extra air from somewhere. possibly the intake manifold gaskets. The extra air coming in fools the computer / o2 sensors to believe it has a lean mixture then it will try to compensate for the lean mixture and add more fuel.

The pre cat sensors running lean does not mean they need to be replaced. They are are detecting a symptom. You need to know why.

If you are using a basic scantool (code only) you need to look up the P0000 code.
I thought this an odd code. I looked it up in my Autotap unit and it does not exist. The lowest # P (powertrain) code I have listed is P0016.

A regular scan tool, won't scan transmission or body codes, mu Auto tap doesn't either. You need a Tech 2 from the dealer for transmission codes.

First some history. The first O2 sensor was introduced in 1976 on a Volvo. California vehicles got them next in 1980, then federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), most vehicles now have multiple O2 sensors, some as many as four!
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.
How does it work? The O2 sensor produces a voltage once it gets hot. The sensor compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.
If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.
Ok – that was a lot of info on what they do and how they work. The next thing to know is that trouble codes relating to O2 sensors are very common. But you really need investigate further before replacing an O2 sensor just because you got that trouble code. Armed with the information above on how often the O2 sensor "flips" back and forth and AutoTap or another scantool that allows you to monitor O2 sensor voltage, you can be certain whether the O2 sensor itself is really the problem. These sensors can be pricey, so don't just replace them the first time you see that trouble code!

The O2 sensors are expensive, diagnose what really is going on.

The O2 sensor should be replaced as a precaution, running hotter indicates a lean air/fuel mixture, the car should be throwing aOBD II code have the codes checked at Autozone to confirn the Oxygen sensor is bad it should read something like bank one sensor or O2 sensor malfunc.