10 Reasons That Cause P0171 and P0174 codes

P0171 and P0174
P0171 and P0174

Every automotive technician in the field most likely will see this code very often. I will help you find the solution to this problem.

There are ten reasons/components that cause P0171 and P0174 codes, which are faulty MAF sensor, a crack air intake tube, clogged fuel filter, weak fuel pump, faulty fuel injectors (clogged or dirty), PCV bad, fuel pressure regulator, vacuum leaks, EGR system (pinto clogged), and oxygen sensors faulty.

The P0171 code is the system too lean on Bank 1. The P0174 codes is the system too lean on Bank 2. The most common symptoms if your vehicle has P0171 and P0174 codes are rough idle, has no power under acceleration and engine will shake at idle (misfiring). Below are the ten reasons/components that contributed this system too lean.

Faulty Mass Air Flow (MAF) Sensor

The MAF sensor is located on the air intake tube. A dirty MAF sensor is usually having very small debris that stuck on the sensor. A very dirty air filter can cause the debris stuck on the sensor. Very important to keep up with the air filter maintenance.

If you have a scan tool and look at the data for MAF sensor. The value for a good MAF sensor is 2 to 7.5 gram for second at idle. As the engine RPM increases the MAF sensor gram per second increases.

A Crack Air Intake Tube

A crack air intake tube can cause the lean fuel trims. When you have a crack intake tube and the air is unmetered. So, what cause the air intake tube to crack? The air intake tube and sometimes suffer from the heat from the engine and due to the age of the air intake tube. The fix for this is to replace it with the OEM.

Clogged Fuel Filter

A clogged fuel filter restricts the full pressure from the fuel pump to the fuel rail. Causing low fuel pressure at the fuel rail. Less fuel pressure which in turn less fuel delivery to combustion chambers. If the fuel filter is restricted, replacement with a new fuel filter is recommended.

There are many reasons that can cause the fuel filter to clog but there are two common causes. One reason is the rusty tank at the gas station. The Second reason is if the vehicle has a metal fuel tank overtime moisture inside the tank create rust. Here is the important tip. When you about to fill up your gas tank make sure there is no fuel truck at the gas station. Because when the fuel truck at the gas station they are filling the big tank causing the rust debris floating around.

Weak Fuel Pump

A week fuel pump causes a weak fuel delivery to the fuel rail. A weak fuel pump is due to the age and worn out internal components. Another very rare reason is corrosion at the external connector. Corrosion causes restriction flow of electrons. For example, a fuel pump is requiring a minimum 12 volts to operate and when the fuel pump connector has corrosion it restricts to 10.9 volts.

Faulty Fuel Injectors

A faulty fuel injector means completely inoperable and not be able to put out a good spray pattern. The fuel injectors are located on the fuel rail. For direct fuel injectors are located direct to the cylinder’s head.

PCV (Positive Crankcase Ventilation)

PCV is usually mounted in a rubber hose grommet in one of the valve covers. During the last stage of combustion some unburned fuel pass through the piston rings this is call a blowby. The main purpose of PCV removes gases from the crankcase and reintroduce into the combustion chamber. The malfunction of PCV can cause shorten the life of the engine and the blowby gases remain in the crankcase causing corrosion.

Fuel Pressure Regulator

The fuel pressure regulator is mounted on the fuel rail. The purpose of the fuel pressure regulator is maintaining constant pressure at the fuel rail for the injectors. A faulty fuel pressure regulator cause the engine runs lean and little rough. The fuel pressure regulator has a diaphragm that control by the engine vacuum.

Vacuum Leaks

The vacuum leaks are usually air that bypass the MAF sensor. Typically, vacuum leaks are crack vacuum hoses, cracks air intake tube, and intake manifold. Vacuum leaks easily identified by viewing scan tool data. In this case, the data on the scan tool STFT Bank 1 = -15 percent and or STFT Bank 2 = -15 percent.

EGR (Exhaust Gas Recirculation) System

The Exhaust gas recirculating system is to reduce the oxides of nitrogen during combustion. The EGR is operated by vacuum and for a newer vehicle is control by electronic. A malfunction Exhaust Gas Recirculation can cause both banks to lean and many other issues. The EGR is usually attaching with the exhaust tube or on the exhaust manifold side.

Oxygen Sensors Faulty

The faulty oxygen sensor can cause the lean code especially this first oxygen sensor (S1B1 and/or S1B2). Because the first oxygen sensor(s) job is to monitor air/fuel ratio. You can see a faulty oxygen sensor on scan tool data. A bad oxygen sensor on the scan tool is O2 sensor Bank 1 is .1 voltage flat. Whereas, a good oxygen sensor voltage should switch between from .1 to 1.0 volt.

The Main Difference of P0171 and P0174 Codes

If a vehicle has 4-cylinder which you will have P0171 which mean bank 1 is lean. Here is the different P0174 code will pop up on your scan tool if the vehicle is a 6 or 8 cylinders (which indicated as V6 or V8). A V6 or V8 has two banks. Therefore, when you have P0171 and P0174 codes you’re working on a V6 or V8 vehicle.

When you are dealing with P0171 and P0174 codes the most common component you want to look are vacuum leaks and MAF sensor. The easiest way is to look the data on the scan tool.

How to Diagnose P0171 and P0174 Codes with Scan Tool

Diagnostic with a scan tool:

  • Step 1: With your scan tool hook up with DLC (Diagnostic Link Connector).
  • Step 2: Go to view data on the scan tool and look for short term fuel trim (STFT Bank 1 and/or STFT Bank 2) look below for more explanation:
    • No Vacuum Leak: Data on the scan tool should between negative 10 to positive 10 percents with engine at 2500 rpm.
      • STFT Bank 1 = -10 to 10 percent
      • STFT Bank 2 = -10 to 10 percent
    • Vacuum Leak: Data on the scan tool is negative 10 percents or more with engine at 2500 rpm.
      • STFT Bank 1 = -12 to – 25 percent
      • STFT Bank 2 = -12 to – 25 percent
  • Step 3: Look at oxygen sensor data, you look at O2 sensor Bank 1 and/or Bank 2. The data on scan tool should be voltage or mil-voltage.
    • A good O2 sensor should be switching voltage from .1 to .9 volt. On some other scan tool may express as milvolts. Such as from 250 to 890 milvolts.
    • A bad oxygen sensor the voltage is not switching from .1 to .9 volt.
  • Step 4: Look at MAF (mass air flow) sensor data. You should have data from .2 to 7.5 or 9.0 gram per second. If you have a bad MAF the data will not be switching.
  • Step 5: Some scan tool have the capability of reading injectors data. More advanced scan tool will more data about all the injectors. Such as injector voltage and/or injector is okay or not.

Tools you will need for this repair and diagnostic:

  1. Autel scan tool or OBD 2 scan tool
  2. Evaporative smoke machine


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