Rick's Free Auto Repair Advice

Fix P0171 code, P0174 code

Diagnose and fix a P0171 code, P0174 code

What is a P0171 code, P0174 code?

First, let’s understand what a P0171 code is. The official definition is P0171 System too Lean (Bank 1). On a V-6 or V-8 engine it’s often accompanied by a P0174 System too Lean (Bank 2). Bank 1 of a “V” engine is the bank that houses cylinder #1. A P0171 code and a P0174 code are the most common trouble codes for all car brands. They rate in the “Top Ten” of the most frequent trouble codes that cause a vehicle to fail emissions testing.

When the air/fuel mixture is properly calculated and burned in the cylinder, there’s no extra fuel or oxygen left in the exhaust. Excessive oxygen in the exhaust indicates that the air/fuel mixture was off the mark.

What causes a P0171 code or P0174 code?

A lean condition can mean the air/fuel mixture contained TOO MUCH AIR for the amount of fuel sent into the cylinder. Or, it can mean there WASN’T ENOUGH FUEL for the amount of air in the cylinder. Both conditions are detected by the oxygen sensor. In a lean condition, the computer see that too much oxygen is left in the exhaust, so it adds more fuel to the mixture. It keeps adding fuel until the amount of added fuel exceeds a preset level set by the car maker. That pre-set level varies with each car maker.

In order to calculate how much fuel to add, the computer looks at inputs from many different sensors:

• Engine coolant temperature sensor (ECT). Engine temperature

coolant temp sensor

Engine coolant temperature sensor

is a critical factor in determining how much fuel to add to the incoming air. A colder engine tends to quench the fire, so the computer adds more fuel to compensate for the colder temperature.

• Ambient air temperature sensor or intake air temperature (IAT).

Intake Air Temperature Sensor

Intake Air Temperature Sensor

Cold and hot air have different densities and thus different amount of oxygen.

• Manifold Absolute Pressure sensor (MAP). The MAP sensor compares barometric pressure to the amount of vacuum created in the intake manifold. MAP sensors are used in speed/density system. When you turn the

Manifold Absolute Pressure Sensor

Manifold Absolute Pressure Sensor

key to start, the computer takes a reading from the MAP before the engine actually starts cranking and creating intake manifold vacuum. The computer uses pre-crank barometric pressure, along with readings from the ECT and IAT to calculate the estimated MASS or density of the incoming air. As soon as the engine fires up, it uses the intake manifold vacuum to determine two things; the efficiency of the engine and the difference in pressure between barometric pressure and manifold vacuum.

• Mass Airflow Sensor (MAF). The MAF sensor uses

Mass Airflow Sensor

Mass Airflow Sensor

a hot wire or hot plate area to determine the density of incoming air. The air rushing across the MAF cools the wire. The sensor monitors how much current it takes to keep the wire or plate at a predetermined temperature. The computer then calculates the density of the incoming air.

What causes a P0171 code and a P0174 code?

It’s really pretty simple: Both codes are caused by either TOO MUCH AIR or TOO LITTLE FUEL that results in a lean mixture. Here’s what causes a too much air condition.

Vacuum leak that results in TOO MUCH AIR

-In a vehicle with a MAF sensor, all incoming air is detected by the MAF sensor. Since the amount and density of air is measured by the MAF, any UNMETERED air that enters the engine will throw off the computer’s air/fuel calculations.

Most common vacuum leaks are

• vacuum line missing, cracked or not connected

• cracked or torn air intake duct that runs from the MAF sensor

P0171 Kia

Air duct that runs from the air filter box to the throttle body

to the throttle body.

• Intake manifold gasket leak

• fuel injector O-ring leaks (the key to finding fuel injector O-ring leaks is to compare the fuel trims when cold versus hot because the o-rings tend to soften and seal better when hot).

• exhaust leaks that allow outside air into the exhaust, making the oxygen sensor see too much oxygen in the exhaust stream.

Low manifold vacuum can cause a P0171 code and a P0174 code

Worn valves, valve seats and worn piston rings

P0171 code

Manifold vacuum gague

or carbon buildup on the valves can cause a cylinder to develop less suction. Use a manifold vacuum gauge to diagnose the condition of your valve

Sensor issues—If the MAF or MAP sensors aren’t working properly or the oxygen sensors are worn or contaminated, the computer will miscalculate the air/fuel mixture.

Most common sensor issues

• dirt or buildup on the MAF sensor hot wire or plate that insulates it and throws off the readings.

• faulty MAF sensor

• Missing, cracked vacuum hose to MAP sensor

• Faulty MAP sensor

• worn or contaminated oxygen sensor(s)

A P0171 code and P0174 code can also be caused by too little fuel

The computer commands fuel based on the readings from the above sensors. But it assumes that the fuel supply and fuel metering systems are in proper working order. The computer has no way of knowing if the fuel pump is outputting low fuel pressure or low fuel volume or if the fuel injectors are clogged.

Most common causes of too little fuel

• clogged or dirty fuel injectors

• low fuel pressure caused by high electrical resistance in the fuel power and ground circuits or relay, clogged fuel filter, a bad fuel pressure regulator or a weak fuel pump

• low fuel volume. A fuel pump can pass a pressure test but fail to deliver adequate volume at higher speeds

• fuel leak inside that tank where fuel pressure and volume is lost between the pump motor and the in-tank pressure regulator or “pulsator.”

How to determine what’s causing the P0171 code and P0174 code

The BEST way to diagnose and trouble shoot these trouble codes is with a scan tool that displays live data. Look at the fuel trim data. If the short and long term fuel trims are high, above 10%, the computer is adding fuel to compensate for TOO MUCH AIR or TOO LITTLE FUEL. If fuel trims are negative, the computer is subtracting fuel. If your engine is equipped with a MAF sensor look at the MAF readings. MAF sensor readings are calculated in grams per second and correspond to the size of the engine. It’s measured at idle. So a good MAF sensor reading for a 3.0L engine should be 3g/S at idle.

Don’t have a scan tool? Here are the things you can do to try and fix a P0171 code and P0174 code.

Clean the MAF sensor. See this post for instructions

Check for vacuum leaks. Look for cracked or missing vacuum hoses or cracks or tears in the air duct from the air filter box to the throttle body. Spray carb cleaner around each fuel injector O-ring and around the engine intake manifold gasket and head gasket to see if vacuum sucks the cleaner into the engine.

Test fuel pressure and volume. But or rent a fuel pressure test gauge from a local auto parts store.

Check voltage drop on the power and ground circuits to the fuel pump.

If you still can’t find the source of the problem and don’t have a scan tool and are willing to throw parts at the problem:

Replace the MAF sensor

Replace the oxygen sensor

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat


Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN