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Emissions Component

How To Fix and Clear OBD-II Trouble Code P0171?

My car failed the smog check. I hooked up my OBD2 code reader and it came up with P0171. What is the exact diagnostics for that? Is it easy to fix? How expensive is it?


OBD-II P0171 Trouble Code

System Too Lean Too Lean (Bank 1)

Here is what a P0171 means, in simple terms

P0171 is pretty straight forward in that it tells us that the engine computer has detected unmetered air. The exact definition of P0171 is System Too Lean (Bank 1). Lean meaning the computer has detected more air in the system than what is required for efficient fuel burning. Bank 1 is the side of the engine where cylinder 1 is located.

The engine control computer uses oxygen sensor data to determine whether or not to trigger a OBD-II P0171. When a trouble code P0171 exists by itself, usually a shop technician will suspect a faulty oxygen sensor. When P0171 exists along with any misfire trouble code(s), a mechanical fault is investigated first.

What caused my vehicle to set a P0171 trouble code?

Trouble code P0171 is indicative of a vacuum leak. We suspect somewhere around the engine there is a vacuum hose that is disconnected, broken or brittle. The break in the vacuum line(s) is allowing unmetered air to enter the intake manifold. If you don't find any problem with vacuum lines, you'll want to check around the PCV (positive crank ventilation) valve and possibly the intake manifold gasket and throttle body. The objective is to find any point where air can enter the engine that is not being accounted for by the engine control computer (ECC). Here are additional problems which may cause a P0171 trouble code.

  • A defective fuel pressure regulator
  • A weak (defective) fuel pump
  • A dirty fuel filter
  • A faulty powertrain control module (PCM)
  • A major/minor vacuum leak
  • Faulty fuel injector(s) (plugged/sticking)
  • Faulty oxygen sensor(s)
  • A faulty mass air flow sensor (MAF)

What symptoms will my vehicle experience when trouble code P0171?

  • Your vehicle check engine light (CEL), Malfunction Indicator Lamp (MIL) or Service Engine Soon light will be illuminated and freeze frame data stored with information regarding engine sensor data at the time when the ECC detected the rich fuel condition. Trouble code P0171 will also be recorded in the ECC ROM memory.
  • Rough and/or high idle may be experienced.
  • Engine pinging may occur during high RPM.
  • Engine temperature may be above normal level.

How can I fix a P0171 problem and where should I start?

The diagnosis for this fault should begin with the inspection of the fuel injection “feedback” system. This includes ensuring the oxygen sensor(s) is working properly, your vehicle's emissions computer is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.

The feed back test will utilize a 5-gas analyzer (usually a smog machine) and propane. A smog check technician should introduce propane to the intake system and ensure the oxygen sensor is reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery at the instant propane is added to the system. This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen we know we have a feedback problem. Then the diagnosis should turn to the part of the feedback system which is defective. Using a voltmeter the technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase. If there is no increase (or too high of an increase for the amount of propane added) we have a defective O2 sensor. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output the technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the computer output terminal to the fuel injectors are lowering “injector pulse rates”.

The additional sensors which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio) are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If the coolant temperature sensor is not operating properly (not sending the correct voltage signal to the Engine Control Module) the ECM will not be able to properly calculate how much fuel to deliver to the combustion chambers. When the engine is cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Ensuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). The MAF sensor measures the amount of air entering the intake manifold. This sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers. Testing of these components are usually done secondary due to the fact that ordinarily if any of these components do not function as required, they usually set their own trouble codes.

As for vacuum leaks... Mechanic shops and smog repair stations use a variety of techniques to find vacuum leaks, from introducing propane near suspected leak areas to using a mechanic's stethoscope to listen for hissing sounds... which most vacuum leaks usually emit. A stethoscope is used because the diagnosis must be conducted while the engine is running. To pin-point to exact location of the vacuum leak you'll want to be able to listen closely to specific suspect areas. This is the safest method to find a vacuum leak, next to conducting a comprehensive visual inspection.

If you choose to use propane, which we don't advise unless you're a trained technician, you'll want to introduce propane through a narrow hose near all areas of the engine where you suspect a vacuum leak to exist. You will notice an obvious RPM increase when propane is delivered near the leak area. We mention only a trained technician should use this method as obviously propane is a flammable gas and utilizing it near any spark source will cause it to ignite. It's a highly effective tool to find vacuum leaks but must be administered with the utmost care and caution.

Before using either propane or a mechanic's stethoscope it would not be a bad idea to simple look over the engine compartment for loose, disconnected or broken vacuum lines, and while the engine is running slightly wiggle vacuum lines near sensors and the intake manifold to detect any increase in engine RPM. A cracked or damaged vacuum line will almost certainly increase RPM when moved around. All the vacuum line needs is a slightly greater opening (break), and the addition of unmetered air into the engine will raise RPMs. Notice an RPM increase and you've found your leak.

At the end of the feedback test it will be know whether your vehicle's lean fuel problem is electronic or mechanical, and the technician can further diagnose the fault. If the fault is determined to be mechanical, the locations which need to be inspected are any and all points of fuel entry and air entry... such as and most predominantly the Fuel Injectors. It is not uncommon for a stuck "closed" or "sticking" fuel injector to cause a lean fuel condition.

  • Check the oxygen sensor(s) for proper operation.
  • Check the ECC input for O2 signal.
  • Check ECT for proper reference, ground and signal voltage.
  • Check MAF for proper reference, ground and signal voltage.
  • Check fuel pressure diverter valve for proper opening and closing.
  • Check fuel injector(s) for stuck closed condition.
  • Check for small/large vacuum leak(s) near intake manifold.

Can I drive my vehicle with a P0171 trouble code and illuminated CEL?

  • You may drive your vehicle with a P0171 however keep in mind the lack of fuel being sent to the catalytic converter is very damaging as it causes the catalyst to burn at extremely high temperatures. The chance of damaging the CAT(s) is very high.
  • The check engine light or malfunction indicator lamp illuminated while P0171 is recorded in the ECC.

posted by SmogTips Support 03-31-2018 08:30 AM

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