Check engine light 1999 Pontiac 150,000 Mass air Flow code was posted on MT2500 Snapon scanner replaced the Censor now code for lean bank one shows ,replaced fuel filter checked Vacumn shows 20 what do you Think?

p0171 is lean exhaust bank one ( passenger side) and p0174 is lean exhaust bank 2 ( drivers side). First we need to look for vacuum leaks, low fuel pressure, Intake air leaks after the mass air flow sensor, pvc system. Because you have both sides lean I would start looking at these spots.

This means the bank 1 sensor is showing that it is running lean this may be due to a bad oxygen sensor, fuel being restricted to the engine, or even a bad mass air flow sensor which could cause the computer to try and compensate air/fuel mixture. O2 sensors are rather pricey and should be checked by a scanner. They should have a minimum and maximum voltage read and if it is out of whack to normal specifications then that is the faulty sensor.

ok heres problem 101 is mass air flow sensor 171 running too rich 174 running too lean can find 429 but it the same lines i believe take out your mass air flow sensor and clean it check it connection if comes back on replace the mass air flow sensor

Have 2000 Buick Lesabre custom every time car gets up to operating temp. the engine misses going down the road , also let car sit and idle and it shuts off after about 2 minutes , my code reads PO171 system to lean bank 1, changed co2 censor, mass air flow censor, plug wires, and plugs.

oxygen sensor failure
P0171 System too Lean (Bank 1) - Read Our Article On Oxygen Sensor Codes For Help With This Generic Check Engine Light Code
P0174 System too Lean (Bank 2) - Read Our Article On Oxygen Sensor Codes For Help With This Generic Check Engine Light Code
hope this is a bit of a push in the right direction ...

PO174= system too lean Bank 2. Good chance oxygen sensor acting up, may need replacement. If it's 4.6L, that's driver side of engine.

--BANK 1 = DTCs P0171 (lean) and P0172 (rich)

-Check intake air system for leaks, obstructions and damage.
-Check air cleaner element, air cleaner housing for blockage.
-Verify integrity of the PCV system.
-Check for vacuum leaks.

I would try cleaning the Mass Airflow Sensor first. A dirty Mass Airflow Sensor is a common problem for triggering PO 171 & 174 codes.

Good luck and keep me posted.

The lean running codes are related to the p-1000 code...

Running lean on bank 1 and bank 2 means your oxygen sensors sense too much oxygen in your exhaust flow. This means either your engine is getting unmetered air (air that the engine does not know is coming in, therefore does not send fuel to compensate for...) (air that your mass air flow sensor does not see). You could have either a bad mass air flow (MAF) sensor (in the intake piping after the filter; this sensor tells your engine how much air it's taking in, so it can send the right amount of fuel through the injectors to properly burn with the air available with as little emissions as possible), or you could also have a cracked/broken intake piping after this sensor which lets air in the intake that the sensor does not measure... A large vacuum leak could also let unmetered air in the engine.

Or... Another possibility here would be low fuel pressure (bad fuel pump, bad fuel pressure regulator, fuel leak), as not enough fuel for the air would also cause the same lean condition.

It could be that some oxygen sensors are bad...

These conditions are hard to pinpoint exactly unless you have a scanner that can read sensor data (MAF Flow, fuel pressure, oxygen sensor values)...

The first thing I'd do would be to check for cracks or damage in the intake piping... If there is none, then it might be a low fuel pressure issue, or sensor problems...

Sounds like a dirty Mass Air Flow sensor.

P0101 : Mass air flow censor, Air intake is blocked or the sensor is nackered.
P0172: System too rich bank 1. There's too much fuel, probably to do with the air fuel mix. If you sort the air flow meter that'll sort the two.

Cheers