What Do P0171 and PO174 Mean?

Code P0171 indicates Bank 1 of the engine has a lean fuel mixture. P0174 indicates Bank 2 of the engine has a lean fuel mixture. Engine Bank 1 is the same side of the engine as Engine Cylinder 1. Engine Bank 2 is the opposite side of the engine. A firing order and cylinder location diagram may be needed to see which side of the engine is Bank 1 or Bank 2. P0171 is triggered by the engine bank 1 “upstream” oxygen sensor and P0174 is triggered by the engine bank 2 “upstream” oxygen sensor. The oxygen sensor alerts your system when the condition is too lean meaning there’s too much oxygen in the exhaust. The symptoms and causes will help you determine the exact issue and how to fix it.

Your bank 1 or bank 2 sensors will be referred to as “sensor 1” or “sensor 2”. Sensor 1 refers to “upstream” sensors, or the ones near the engine and before the catalytic converter. Sensor 2 is “downstream” or after the converter. These upstream sensors verify the correct air/fuel ratio is entering the engine cylinders by measuring the oxygen levels in the exhaust gases leaving the cylinders. This is done by a continual but changing voltage signal to the computer. A continual high voltage reading indicates a rich fuel mixture, not enough oxygen. A continual low voltage reading indicates a lean fuel mixture, too much oxygen. The optimal ratio is 14.7:1 (air:fuel). When your powertrain/engine control module receives a reading that indicates there’s too much oxygen and the ratio is incorrect, it sends an error code P0171 or P0174. Depending on the problem, it may send both codes if the issue is causing a lean condition on both engine banks. It’s important to note here, the very nature of the oxygen sensor sending these codes, without any other oxygen sensor fail-codes tells us the sensor is working as it should. What makes this code tricky is if this code appears by itself, there is NOT a monitored component that has failed, or it would be getting a code as well. In short, the engine is running lean, but the computer cannot tell why.

P0171 and P0174 Symptoms

In many instances engine performance issues are not exhibited. However, some lean situations can cause your vehicle to have these performance problems:

  • Surging or hesitating acceleration
  • Misfires (which will usually set a misfire code)
  • Spark knock or detonation
  • Reduced power
  • Rough idle

More commonly, you’ll only be alerted by the presence of an illuminated CEL. Look for one of these common causes to determine how to solve the problem code.

The first thing to determine is what other codes are present? If either code arrives (or both) at the same time as, say, a Mass Airflow Sensor code, it’s a logical step to assume the MAF sensor may be causing the lean condition. If either P0171 or P0174 is appearing by itself, or both codes are stored, you have a problem that requires more detective work, and you must look beyond sensors and items monitored by the engine’s computer.

Common Causes

There are many minor or major repairs that could create this issue. Start with the easiest issues and work your way through to identify the cause of your problem code. Lean conditions are often caused by air entering the system somewhere where it cannot be monitored. Two main areas for this would be the intake system, usually after the MAF sensor, or a vacuum leak somewhere in the intake. Start by checking the intake boot from the MAF to the throttle body. Inspect for cracks, loose hose clamps, or anywhere air could enter. Next, check over all the vacuum hoses on the engine for any cracks or breaks in the hose. While the engine is running, listen carefully as often you can hear a major vacuum leak from the sound of air being sucked through a break. Check the entire PCV system of hoses for leaks or cracks as well.

If nothing pops up here, there are other major issues that could be occurring. One is a vacuum leak due to a failed intake manifold or throttle body gasket. This will cause a major vacuum leak and trigger a lean condition. Another possible issue is improper fueling, which could be caused by low fuel pressure (potentially a fuel pressure regulator, or clogged pump strainer in the tank, or weak pump), or weak / plugged / misfiring fuel injectors. Its important to note that usually fuel issues such as these will often trigger a misfire code as well – either random or on particular cylinders.

How to Fix P0171 and P0174

While inspecting the intake boot thoroughly, another option is to stop at the MAF sensor itself. MAF sensors can get dirty or oiled and send out a false signal, but usually this is also indicated by a MAF trouble code as well. This is particularly true if you use oiled air filters that have too much oil on them, causing contaminants to cling to your MAF sensor. Once you find your sensor, you can remove it and clean it with MAF cleaning spray, available at AutoZone.

Once the sensor is cleaned and working properly, and all your hoses and intake have been checked over, a vacuum test on the engine while running is the next step. A vacuum diagnostic gauge can be hooked up and used, or a professional shop can help at this point. If the vacuum is out of parameters or fluctuating wildly, it is likely there is a hidden breakage in a hose, or the intake manifold gasket is suspect.

Once you find the solution and repair or replace the appropriate part, check your vacuum and reset your trouble code to test whether you’ve fixed the issue or need to look for further vacuum leaks.

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