What causes a P0171 code From an automotive technician perspective?

I know when you see the check engine light is on a while you are driving or when you are starting a vehicle as it can frustrate. I know this because I have experienced it for myself. You can drive your vehicle when this P0171 code is on, as long as the vehicle is not stumble or have a hard time to stay running.

In general, what causes P0171 code are the following, a malfunction mass air flow sensor, vacuum leaks, bad oxygen sensor and bad injector. P0171 code, which simply means bank 1 fuel system is too lean. According to many technicians in the field, “fuel system is too lean” is powertrain control module (PCM) subtracting fuel or not enough fuel get into system according to manufacture specification.

Mass Air Flow Sensor Primary Purpose

The function of mass airflow is to monitor the air volume going into the intake. The mass airflow/intake air temperature sensor contains a hot wire, a cold film, and a thermistor. The sensor is in the intake passage. The resistance of the hot wire and thermistor change due to intake air temperature and airflow. The circuit in the MAF sensor controls the current to keep the hot wire at a set temperature. The current is converted to voltage in the control circuit, then output to the PCM.

What Causes/Triggers P0171?

As a general rule, an unplugged or a malfunction mass airflow (MAF) sensor will trigger P0171. Intake vacuum leak can trigger P0171. A vacuum leaks is the unmetered air flow to the engine, most of the time is the air by pass the mass air flow sensor.
There are a number of components can cause P0171 code to trigger, however; these are the most common components’ culprit for P0171 code:

  1. Unplugged mass airflow sensor
    • You can drive the vehicle if the mass airflow sensor is unplugged, but you will have poor acceleration capability. On most vehicle, you will be able to drive the vehicle for a short distance.
  2. A malfunction mass airflow sensor
    • In a malfunction mass airflow, you will see no reading value on scan tool.

Can a bad fuel pump cause P0171?

In general, a bad fuel pump can cause p0171 code to turn on. Let’s define a bad fuel pump. A bad fuel pump here is what I usually define working but do not put out enough gasoline volume according to specification. A bad fuel pump in this case is pumping 35 psi of fuel pressure, however; a good fuel pump usually is around 55 psi of fuel pressure.

The below picture is the most common fuel pump & fuel pump assembly for American made vehicles.

Reasons cause a bad fuel pump

One particular reason causes a bad fuel pump is worn out because either working for so long. I have encountered many times a vehicle with 120,000 miles (193,121.28 km) with a bad fuel pump either was towed in or died while driving. Another reason, I also have many vehicles that fuel pump did not produce enough fuel pressure to manufacture specification. The most common problem for fuel pump that cause P0171 is the fuel pump do not produce enough fuel pressure to manufacture specification. For example; a vehicle manufacture specification required fuel pressure at 55 psi, and you measure it with the fuel pressure gauge is 35 psi.

Is it Ok to drive with P0171 code?

There are about 80 percents you will be able to drive your vehicle if you have P0171 code. You are okay to drive your vehicle for certain amount of time, however; it is recommended if you have this code please bring your vehicle to repair shop as soon as possible to prevent any further damage to your vehicle.

Tools requirement for diagnosing P0171 code

  1. On-board Diagnostic 2 (OBD2) Scan tool
  2. Fuel pump pressure test kit
  3. Automotive smoke machine
  4. Socket sets
  5. Safety glass/Safety glass wear
  6. Automotive industry heavy-duty gloves


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