I have a 2005 mercury Monterey van with a check engine light on. Autozone diagnosed either a mass air flow sensor or O2 sensor problem. I tried cleaning the air flow sensor with air flow sensor cleaner but check engine light is still on. The codes displayed are P0171 and P0174

ok lets start with the codes they say your getting. P??? which 1's and how many. there are few o2 sensors, are they next to the Exhaust next to the converter? MASS AIR FLOW -MAF use this, for now i am guessing, until i see the correct codes your are getting. this could be the 1 you may have below, please read it........

• P0103
  Mass Air Flow Sensor Circuit High Input
• Possible causes
• Faulty mass air flow sensor
  - Intake air leaks
  - Dirty mass air flow sensor
  - Dirty mass air filter
  - Mass air flow sensor harness is open or shorted
  - Mass air flow sensor circuit poor electrical connection
• Before replacing the mass air flow sensor, try replacing the air filter and cleaning the air flow sensor with low compress air or mass air flow sensor cleaner. Reset code and drive vehicle. If the code comes back, it may be necessary to replaced the mass air flow sensor.
• possible symptoms Engine Light ON (or Service Engine Soon Warning Light)
  - Engine stall
  - Engine running rough
  - Excessive fuel consumption
  - Excessive smoke
• the code is generated when An excessively high voltage from the sensor is sent to Engine Control Module (ECM).

2003 NISSAN Altima Sedan OBDII Codes List

O2 sensor

I found a YouTube video, MAF Sensor Cleaning in Slow Motion-rzrDIRZ-ccs.webm, that showed me how to remove the mass air flow sensor and spray clean it with CRC mass air flow sensor cleanser. I bought the kit (single use of Mass Air Flow Sensor Cleaner and Throttle Body and Air-Intake Cleaner at Walmart for $7.99 in store price) and cleaned the mass air flow sensor. Then I reset the check engine light by removing and replacing the negative battery terminal. It worked. The check engine light has stayed off. I have a 2005 Toyota Matrix.

Air Flow Sensor (MAF)---was changed why? Was it even tested in any way?

The PO 171 -- is an engine right side Oxygen Sensor reporting a lean condition

You have a repair shop diagnose that

Doesn't mean the sensor is bad
Could be a Vacuum Leak any number of places

Hopefully you changed the O2 Sensors at 100,000 miles
during Preventative Maintenance

have it cked maybe nothing or something maybe wrong lot of times it could be o2 sensor or mass air flow you maybe using more fuel than you want to

Only way to find the exact problem is to have the engine trouble codes read with a scanner. Most larger auto parts chain stores will read the codes for free and give you a printout. Then, you can inquire about the specific codes and parts needed to fix it.

Your code is a lean engine code. Need to have the O2 sensor checked. Possible vacuum leak. Common problem with these cars is the massairflow sensor. This can be verified with the use of a scan tool. More than likely you have a massairflow problem.

I have a 95 Trans Am myself, with a worked 383 lt1 with full bolt ons. etc Your using the factory tune with that setup? While the check engine light might not be on because of the factory tune i would strongly suggest at the least, getting a mail order tune, and if you can, a dyno tune. You're not seeing the full potential of your setup the way it is now. Are you using an aftermarket mass air flow sensor? The stock mass air flow sensor flows plenty for most setups. Aftermarket sensors can cause driveability issues and many suggest staying away from them.
Are you able to scan the computer to retrieve the codes? It's impossible to diagnose the check engine light without knowing why it's on.

Hi,
I don't know if you have a 4 or 6 cylinder engine.
Here are a couple of links to Autozone where you can look at the part and identify it on the top of your engine. "A picture's worth a thousand words". Hope this helps... Good luck!

If 4 cylinder:
http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?categoryDisplayName=Engine+Management&fromType=parts&fromString=search&parentId=59-0&currentPage=1&filterByKeyWord=mass+air+flow+sensor&isSearchByPartNumber=false&navValue=15900151&categoryNValue=15999999&fromWhere=&itemId=151-0&displayName=Mass+Air+Flow+Sensor&searchText=mass+air+flow+sensor

If 6 cylinder:
http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?categoryDisplayName=Engine+Management&fromType=parts&fromString=search&parentId=59-0&currentPage=1&filterByKeyWord=mass+air+flow+sensor&isSearchByPartNumber=false&navValue=15900151&categoryNValue=15999999&fromWhere=&itemId=151-0&displayName=Mass+Air+Flow+Sensor&searchText=mass+air+flow+sensor&vehicleIdForService=65220189&_requestid=9624

It might but what exactly are the codes. Before you change the Mass air flow sensor, try cleaning it first with a can of mass flow cleaner.
If your running a oil K&N filter, there could be some oil residue on the sensor. Start with that and change the filter or clean it if it's a K&N.
The O2 sensor's could be bad also, you really need the code's to be sure what sensor is giving you the faulty signal.
Good luck and hope this helps.