Your morning is going great and you’re leaving right on time for work—until you go to start your car and your Check Engine light is on and the engine is running rough. You grab your handy OBD II scan tool or code reader and find out you have a P0171 code.
What does code P0171 mean?
“Code P0171 – system too lean bank 1” means there is too much oxygen or too little fuel in the exhaust, which is referred to as a “lean” condition, or a lean or “low fuel trim” code.
This can be caused by something like a disconnected hose or vacuum leak, which means there’s more air than usual, or because your fuel system is weak. P0171 is a common trouble code in many vehicles.
A “bank,” as in “bank 1,” refers to the side of the engine where each of your car’s cylinders is found. Bank 1 is where cylinder number 1 is located, and where you’ll find the cause of the code.
For your car to run the way you would expect it to (read: no check engine light and zero signs of starting or running issues), its engine needs a certain amount of air and a certain amount of fuel. This mixture ratio is pretty precise so if it’s off even by a small amount, you could experience the symptoms caused under a P0171 engine code.
If that air-fuel ratio is out of balance, you’ll either notice that something is off, or your car’s computer will detect it first. What is that correct ratio? To function optimally, an engine needs a mixture of 14.7 parts air to 1 part fuel (14.7:1).
With the help of oxygen sensors, your car’s engine or powertrain control module (ECM or PCM) is responsible for measuring the air-fuel mixture and adjusts the amount of fuel depending on the amount of air.
If there is too much air, the ECM or PCM overcompensates with larger fuel injections, which triggers the P0171 code.
How serious is a P0171 code?: Is it OK to drive with P0171?
A lean fuel trim code like P0171 causes performance problems that can lead to costly damage, so it’s best to avoid driving with it. These problems can include bad fuel economy, poor engine performance, and even internal engine damage.
What are common P0171 code symptoms?
Code P0171 symptoms can include:
- Your Check Engine light is on
- Trouble starting your engine
- Rough running engine
- Trouble idling or accelerating
- Misfiring, stalling, or losing power
Code P0171 causes and how to diagnose them
P0171 causes can range from easier-to-find issues to more complex problems and can vary among vehicles.
Watch this video for expert mechanic help diagnosing P0171 code causes, and keep reading for more details:
Video: Check Engine Light? System Too Lean – Code P0171, P0174
Diagnosis difficulty: Easier
P0171 causes to look for and diagnose first:
Disconnected or leaking hoses
- Airbox snorkel: Find the airbox snorkel that connects to the engine. Grab and shake it so you can feel if anything is loose. If the snorkel appears to be okay, you can also check this area for any other disconnections.
- Underneath the engine cover: Look for anything that looks disconnected for any open ports that seem like they’re missing a hose. Also, check for any blocked ports.
- Whistling noises while your car is running: Start your engine and listen carefully for a whistling or rushing air sound while you stay away from any moving components under the hood. Hear anything? Start looking in the area where you’re hearing noises for a leak, open port, or loose hose. The code’s cause could be something as small as a pinhole in one of the lines, so you want to be thorough when trying to diagnose code P0171..
Dirty, faulty, or damaged mass air flow sensor or air filter
- Mass air flow sensor: Check for any debris. You may even find a leaf that got through and is blocking the mass air flow sensor.
- Air filter: Look for any holes or debris. If you find any holes, you’ll need to replace the air filter. While you’re looking in this area, you can also check for any debris that may have gotten in past the air filter and needs to be cleaned out.
Diagnosis difficulty: Advanced
Tools needed for diagnosing advanced P0171 code causes:
- OBD II scanner or code reader
- Carburetor or throttle body cleaner
More complex problems to check for:
Unsealed, disconnected, or damaged engine intake parts
Make sure everything in the engine intake system is sealed and connected, and check for leaks.
- Gaskets: Check the gaskets in between the plenum and gasket heads for leaks and cracks. After trying to find these types of issues on your own, you could take your car to a professional mechanic for further inspection.
- Purge valve seal: If your engine is sounding rough, there could be a large leak causing the P0171 code. You can also check for a bad seal on the purge valve.
How to test for leaks:
- While the engine is running, spray carburetor or throttle body cleaner around the areas where you think there’s a leak. The engine will either run smoother or rougher and can help you figure out where a leak is located.
- Have someone watch the gauges for you while you spray: If the gauges fluctuate, that’s another indicator that should help you find the leak.
- Graph your engine speed: If you have a scan tool or code reader that can graph engine speed, you can watch the reader while you spray carburetor cleaner on the intake gasket or surrounding area. If the graph drops or spikes in revolutions per minute while you spray, look closely for a leak where you’re spraying at the time of that change.
Other leaks to check for
Low fuel rail pressure and plugged or blocked fuel injectors, filters, or pump
- Fuel rail: Check the fuel rail’s pressure. If the pressure isn’t high enough, that’s a sign your car isn’t getting the amount of fuel it needs, which can trigger the P0171 code.
- Fuel injectors, filters, or pump: Check to see if these parts are plugged up or blocked, which would keep the right amount of fuel from reaching the engine.
Bad oxygen sensors
O2 sensors monitor any unburned or fuel that gets into the exhaust. If an oxygen sensor isn’t sending the correct signal to the computer, that will cause an issue and can trigger a P0171 code.
Other sensors to inspect
You can also check for faulty coolant temperature sensors and other engine monitoring sensors.
How do you fix code P0171?
If you’ve diagnosed what’s causing your car’s P0171 code, but it’s not something you can repair on your own, you’ll need to take your car to a professional to fix the trouble code. It’s important to work within your comfort zone when it comes to diagnosing car issues and fixing them.
If the cause has a simple solution that you know you can tackle on your own, like reconnecting a disconnected hose, make sure your Check Engine light goes off after reconnecting it, your car’s rough idle is gone, and that everything seems to be working normally.
You can also reference repair manuals for your vehicle for troubleshooting and repair help.
Learn about other check engine and trouble codes when you watch our latest videos.
Blog posts to read next:
- What Is OBD-II And How Does It Work?
- What Is An O2 Sensor?
- What Is Fuel Injection? How Does It Work?
- 6 Things Shady Mechanics Don’t Want You to Know