The check engine light is on and the diagnostic code is po171 - system too lean bank 1. Is this an oxygen sensor problem and how much should I expect to pay for replacement if needed?

All of these p1131 p1151 p0171 p0174 codes indicate a problem with a lean condition. This engine is known for vacuum leaks, so before you start throwing oxygen sensors at the problem, it pays to perform some diagnostics.
Start by cleaning the MAF sensor. Spray the sensor with CRC MAF sensor cleaner (even though Ford tells you to replace it because it can't be cleaned). Next, check the plastic vacuum lines, especially where they meet with rubber boots. If they are cracked or the rubber is swollen, replace the line. Then check the rubber elbow that holds the PCV valve. That elbow is a know failure point. In fact, the entire PCV tubing is a trouble spot on these engines. (thanks Ford).
After you've checked for vacuum leaks, check the operation of the oxygen sensors. The easiest way to do that is to disconnect the sensor from the harness. Connect one lead of your digital voltmeter to the black wire on the sensor and the other lead to battery negative. Then start the engine and add fuel (either propane, starting fluid, or carb cleaner) into the throttle body to force the mixture to rich. You should see a corresponding increase in voltage as you add fuel (from 0 volts to slightly less than 1 volt). Start and stop the fuel enrichment and watch the meter to see if the volts drop as you stop adding fuel. If you see no change, the sensor is dead. Replace it.
You can also use the fuel enrichment procedure to locate intake gasket leaks.
P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1
P1151P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2
P0174 system lean bank 2
P0171 system lean bank 1

bank 1 sensor 1 lean code. Check vacuum lines first. Could be egr leak also. Most vacuum lines run on the back side of the engine on these. Most common fail is the PCV 90 degree elbow. On the passenger side.

Most likely a Oxygen sensor.

Google the Codes
OBD PO171 etc & print a code chart
They are Oxygen Sensors-lean on both banks
They should have been changed at 100,000
miles on all your vehicles.

PO171 OBD2 Code basically this means that an oxygen sensor in bank 1 has 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. The P0171 is one of the more common trouble codes.

his code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code.

DTC P0171 - System Too Lean (Bank 1)
Basically this 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.

Note: This DTC (Diagnostic Trouble Code) is very similar to P0174, and in fact your vehicle may show both codes at the same time.

I would suggest that you take a look at the front oxygen sensor. Typically speaking the oxygen sensor causes this code to appear. Here is the possible cause list from the manual for your car.

Trouble Code: P0171 - Fuel System Lean (Bank 1)

Possible Causes:
Air leaks in intake manifold, exhaust pipes or exhaust manifold
Base engine mechanical problem causing a lean A/F condition
Fuel control Sensor is out of calibration (e.g., ECT, IAT or MAP)
Fuel delivery component fault (clogged filter, low fuel pressure)
O2 element is contaminated, deteriorated or it has failed
Vacuum hose is disconnected, broken, leaking or loose

Hope this helps (remember to rating this answer ans leave us a testimonial comment). Thank you for using FixYa.

DTC P0171 - System Too Lean (Bank 1)
Basically this 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.

Note: This DTC (Diagnostic Trouble Code) is very similar to P0174, and in fact your vehicle may show both codes at the same time.

I would suggest that you take a look at the front oxygen sensor. Typically speaking the oxygen sensor causes this code to appear. Here is the possible cause list from the manual for your car.

Trouble Code: P0171 (2.5L V6 Auto) - Fuel System Lean (Bank 1)

Possible Causes:
Air leaks in intake manifold, exhaust pipes or exhaust manifold
Base engine mechanical problem causing a lean A/F condition
Fuel control Sensor is out of calibration (e.g., ECT, IAT or MAP)
Fuel delivery component fault (clogged filter, low fuel pressure)
O2 element is contaminated, deteriorated or it has failed
Vacuum hose is disconnected, broken, leaking or loose

Hope this helps (remember to rating this answer ans leave us a testimonial comment). Thank you for using FixYa.

Code Po171 indicates a lean condition on cylinder bank #1 of the engine. This is the drivers side of the engine. I would suggest replacing both front oxygen sensors as a pair to start. An oxygen sensor has a life span of about 5 years and yours are now 9 years old if they are original. When they get older they are slow to respond to changes in the oxygen content of the exhaust and can trigger a code Po171. If you continue to have the same code set after the replacement of both oxygen sensors then it is time to find out why your engine is running lean. Some of the most common causes are an ignition misfire and a vacuum leak. I have also seen Fords EGR flow sensor (known by Ford as Delta Pressure Feedback sensor) to cause problems like that as well. A good mechanic with a scan tool can usually spot a problem like that in a few minutes.

Sounds like you have low fuel supply or a leak in intake system causing lean codes, check fuek pressure and volume first. Cat converter effeciancy could be caused by the lean codes, purge system sounds like solenoid failure, check for high resistance accross purge vent solenoid.

i think its the oxygen sensor (pre cat) in exhaust