P0171 Code: System Too Lean Bank 1
The P0171 code stands for “ System Too Lean Bank 1”. A lean condition means there’s too much air entering the engine, compared to the amount of fuel. The perfect air-fuel ratio should be 14.7 parts of air to 1 part of fuel. If the air-fuel ratio is incorrect then your car’s gonna experience some issues.
Location
Typically, "Bank 1" is the side where cylinder 1 is located. If you’re having trouble finding it, you can refer to a repair manual.
Sometimes diagnosing the exact issue when dealing with the P0171 code can get tricky. However, this is when an OBD2 scanner can come in handy. Check to see if you're getting any other codes besides the P0171. If you do, then that’s your starting point. However, if you’re only getting P0171, it could get a little more tricky. But let’s go ahead and break down the potential causes.
Vacuum leak
The first and most common cause in my experience for the P0171 is a vacuum leak. A vacuum leak is when air enters the engine that is not sensed by the MAF or the MAP sensor. Here’s a list of components where a vacuum leak may occur:
Anything after the MAF sensor (Mass Airflow Sensor) that is torn/cracked
Loose hoses
Torn intake manifold gasket
Cracked intake manifold
Torn vacuum hoses
Stuck-open PCV valve
Stuck-open EGR valve
Leaking purge valve
Ruptured diaphragm inside the brake booster
Torn brake booster hoses
MAF Sensor
The second cause could be a bad mass airflow sensor. The MAF sensor measures how much air goes into the engine. It helps the engine control module (ECM) decide how much fuel to add for proper combustion. If the MAF sensor is faulty, it can provide wrong air readings, and trigger the P0171 code. Also, sometimes the sensor can just be dirty. Before replacing it, try spraying it with a MAF sensor cleaner and see if it will solve the issue.
Fuel issues
The third cause could be fuel issues. If there are fuel delivery issues in the system it can cause your car to run lean because there will be less fuel and more air, and as mentioned earlier an incorrect air-fuel ratio will make your car act up. Some components that can cause this issue within a fuel system include a faulty fuel pump, a dirty fuel filter, faulty fuel injectors, a faulty fuel rail pressure sensor, or a restriction in the fuel supply line. You can perform a fuel pressure test if you suspect fuel delivery issues in your vehicle. I’ll leave a fuel pressure gauge that you can use in the description below.
Keep in mind that the next 3 causes that I’ll talk about are more rare in my experience when it comes to the P0171 code. However, it’s still important to talk about them.
Exhaust Leak
The fourth cause of the P0171 code is an exhaust leak. To be more specific an exhaust leak before the upstream oxygen sensor. This can allow outside air to enter the exhaust system and trick the oxygen sensor into believing that there’s more air than there actually is. This can trigger the P0171 code. Common locations for exhaust leaks include:
Cracks in the exhaust manifold
Exhaust manifold gasket
An exhaust pipe leading to the oxygen sensor
Oxygen Sensor
The next cause is a faulty oxygen sensor. If the O2 sensor is malfunctioning, it can provide inaccurate readings, indicating that there is too much oxygen in the exhaust system causing a lean condition. This incorrect data leads the ECM to adjust the air-fuel ratio inaccurately. Now, it’s important to note that this sensor is gonna be located on bank 1.
ECM
The last cause for the P0171 is a bad engine control module. This won’t happen often but it’s still a possibility to consider.
