2001 Chevy Impala 3.8L. P0171 lean fuel code. I just had my mass airflow sensor, downstream O2 sensor, And fuel tank pressure sensor replaced. I also had a few injection service. Before this my car was not running at all once it got up to running temperature it would stop. After taking my car home for mechanic I had a rough idle especially when I started up before the engine heat it up. Any advice on what to do?

A P0171 means that your Toyota is having trouble with a dirty or faulty mass air flow (MAF) sensor, or a vacuum leak downstream of the MAF sensor but this code can also signify a bad O2 sensor in the exhaust on bank 1.(the one closest to the engine on bank 1)
Also, P0171 code can be tripped by a plugged fuel injector, by a dirty fuel filter causing low fuel pressure, or even an exhaust leak before the first O2 sensor.Most of the time, the best way to respond to a P0171 is to try cleaning the MAF sensor and the lines running to and from it. This will often resolve this issue.Next is change the oxygen sensor in bank one and closest to the engine.

code p0171 refers to system too lean, bank 1===causes --intake/exhaust leak --AIR system---MAF/VAF sensors--fuel pressure/pump--injectors---HO2S sensor
as the HO2S sensor is the first sensor to inform the ECM of the mixture setting problem I would consider changing it first. Have the fuel pressure checked as there may be a problem with the pressure regulator in the manifold and lastly have the injectors checked for tip blockages.

P0171 is a lean bank one code. It is usually always a vacuum leak and is usually not an O2 sensor or a MAF sensor. The first step shoul have been to check for vacuum leaks.

What engine do you have?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

for p0171, try:
In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect all vacuum and PCV hoses, replace if necessary
Check for a dirty fuel filter and proper fuel pressure
for p0420, try:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.

Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).

Inspect the downstream heated oxygen sensor (HO2), replace if necessary

Replace the catalytic converter

P0171 system too lean bank 1
P0174 system too lean bank 2
links to information on the codes

it means that the O2 sensors in your vehicle detected a lean condition in their respective banks. this could be caused by a dirty MAF (mass air flow) sensor or a vacuum leak downstream from the MAF sensor

sometimes you can clean the MAF with electronics cleaner or brake cleaner but be sure to take it off the vehicle first. be careful not to damage the MAF while spraying the cleaner, and allow it to dry completely before installing it on the vehicle.

If your O2 sensors are malfunctioning, the main symptom would be that the Check Engine light will come on. On some vehicles, an O2 sensor circuit shorted to ground can cause a no-start problem.
If the "Check Engine" or "Service Engine Soon" light is on and there is a code for the O2 sensors, I would like to also advise you that many O2 sensor codes are not caused by the O2 sensors themselves. In many cases, the O2 sensors are only REPORTING the problem. for example, a code P0171 "Oxygen Sensor Lean" code is rarely ever caused by the O2 sensor. It is usually caused by a vacuum leak or a failed Mass Airflow sensor (MAF) or Manifold Absolute Pressure sensor (MAP) or a bad fuel injector. In these cases, repairing the real reason that the engine is running lean will fix the O2 Sensor code without replacing the O2 sensor.

P0171 is a lean fuel condition . The computer reflashed has nothing to do with it . Vacuum leak intake manifold probably . If both P0171 & P0174 were back could be fuel pump low flow or restricted fuel filter . or O2 sensor problem or mass airflow sensor. Suggest you take it to a qualified repair shop to have it diagnosed .
Ford 150 P0171 P0174 Trouble Codes vacuum leak water test

Diagnosing Faulty Mass Air Flow Sensor P0171 P0174 Lean Codes

LEAN TROUBLE CODES (P0171 or P0174) A code P0171 or P0174 indicate the engine is running lean. This means there is too much air and/or not enough fuel. You can confirm the engine is running lean by looking at Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Normally, STFT and LTFT should be plus or minus 8 from zero. If STFT and/or LTFT are more than about +10 or higher, it indicates the engine is running LEAN. Negative numbers (-10 or more) indicates the engine is running RICH. A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)

______________________________-- Causes A code P0171 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 may 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 downstream of the MAF sensor.

Possible Solutions Possible solutions include:

• In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
• Inspect all vacuum and PCV hoses, replace if necessary
• Check for a dirty fuel filter and proper fuel pressure

P0171 - System Too Lean (Bank 1) indicates a bad reading from an O2 sensor or bad MAF sensor.
CausesA code P0171 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 may 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 downstream of the MAF sensor.

Possible SolutionsPossible solutions include:

• In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
• Inspect all vacuum and PCV hoses, replace if necessary
• Check for a dirty fuel filter and proper fuel pressure

Hi
171 system too lean
174 system too lean
2195 - 02 ozygen sensor signal stuck lean Bank1 Sensor 1
2197- 02 ozygen sensor signal stuck lean Bank 2
Sensor 1
The 2 front 02 sensors are located on the front of
the exhaust manifold.
Should solve problem and increase gas mileage.