2002 Mercury Mountaineer CODE P171 & p172

P0171 & P0172 means that an oxygen sensor
in bank 1 detected a "lean" condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.

Causes:
A code P0171 & P0172 may mean that one or more of the following has happened:

* The MAF (Mass Air Flow) Sensor is dirty or faulty.
Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
* There could be a vacuum leak.
* There could be a fuel pressure or delivery problem.

I WOULD CLEAN THE MAF SENSOR WITH "SPRAY" ELECTRONICS OR BRAKE CLEANER. MAKE SURE IT IS "COMPLETELY" DRY BEFORE YOU REINSTALL IT.
CLEANING THE MAF USUALLY SOLVES THE PROBLEM 95% OF THE TIME.
The MAF sensor is located right near your air cleaner box. Right on the black rubber air flow tube, it is black and has "Air Flow" marked on it with an arrow pointing to the flow direction.

Possible other solutions include:

* Inspect all vacuum and PCV hoses, replace if necessary

* Inspect fuel lines for cracks, leaks, or pinches
* Check the fuel pressure at the fuel rail
* Check the fuel injectors, they may be dirty. Use fuel injector cleaner or get them professionally cleaned/replaced.
* Check for an exhaust leak before the first oxygen sensor (this is unlikely to cause the problem, but it is possible).
I hope this information helped.
Good Luck;
Rick