Check Engine Light DTC Code P0171 and P0174: Fuel Trim System is Too Lean

OBD-II fault code P0171 means “Fuel Trim System is Too Lean” in bank 1. Code P0174 indicates the same issue for your bank 2 air-to-fuel ratio. Bank 1 and bank 2 are names indicating the banks of cylinders and corresponding components on either side of a V-type engine, with bank 1 being the side that contains cylinder 1, and bank 2 indicating the opposite side. In four-cylinder or straight-six engines, there will not be a bank 2, since there are not two sides to the engine itself. These two codes will often appear with P0170 and P0173, which indicate Fuel Trim Malfunctions in bank 1 or bank 2. When you see P0171 or P0174 codes, it means the powertrain control module in your vehicle is receiving information that indicates your engine’s air-to-fuel mixture contains too much air and not enough fuel. This is referred to as the engine running lean.

P0171 or P0174 Code Diagnosis

When you experience a P0171 or P0174 diagnostic trouble code, your vehicle may be experiencing failure in one of a few areas:

• Dirty or faulty mass air flow sensor
• Vacuum leaks from the intake manifold gaskets, vacuum hoses, or PCV hoses
• Clogged fuel filter
• Weak fuel pump
• Outdated control module
• Clogged or contaminated fuel injectors

An engine misfire, fuel pressure issue, or defective oxygen sensor can lead to your catalytic converter not functioning efficiently, or the exhaust system or catalytic converter itself may have failed, and you may see additional codes that can help narrow down your diagnosis.

P0171 or P0174 OBD-II Code Symptoms

There may or may not be noticeable signs or symptoms of a P0171 or P0174 code besides a Check Engine light.

Symptoms of a P0171 or P0174 OBD-II Code can include:

• Lack of power
• Coughing/Sputtering
• Misfires
• Knocking
• Pinging
• Trouble idling
• Difficulty turning over

If you’re comfortable diagnosing the issue yourself, or if you have other codes related to a misfire or oxygen sensor problems, you may be able to determine the cause of your P0171 or P0174 code with a physical inspection.

Unfortunately, this is a difficult problem to diagnose, and while replacing the oxygen sensor, also called an O2 sensor, may seem like an easy fix, it won’t always solve the problem. If you suspect this problem could be related to your fuel system, you can perform a fuel pressure test on your vehicle with the fuel pressure tester available in our Loaner Tool program. We also have a video on how to use the fuel pressure testing tool in our How-To Hub. This may help eliminate issues with the fuel pump or fuel filter to help you complete your diagnosis.

Whether you notice any symptoms or not, it is not recommended that you drive a vehicle with this code until the problem is diagnosed.

Risks of Driving with a P0171 or P0174 Code

Your vehicle may still run, but driving has the potential to make the problem worse or cause damage to other systems. When the engine is running lean or misfiring, it has the potential to cause damage to the pistons, crankshaft, bearings, and more.

If your Check Engine light is on, stop by any O’Reilly Auto Parts location where our Parts Professionals can scan and diagnose your illuminated Check Engine light for free with our exclusive O’Reilly VeriScan code reading service. An O’Reilly VeriScan provides a detailed report of possible causes and solutions or repairs for your illuminated Check Engine light. The O'Reilly VeriScan report, along with the O’Reilly Auto Parts How-To resources can help you understand what parts, tools, and services you need to clear your DTCs and Check Engine lights. If you aren’t comfortable performing any necessary repairs yourself, we are also happy to recommend a local repair shop.