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.
What Does Code P0174 Mean?
Diagnostic trouble code (DTC) P0174 stands for “System Too Lean (Cylinder Bank 2).” The code indicates that your car’s computer has detected a lean running condition.
The error codes for lean conditions are P0171 and P0174. The former is triggered when a lean condition occurs in cylinder bank 1, while P0174 appears when it occurs in cylinder bank 2.
Even though the Engine/Powertrain Control Module (ECM/PCM) is built to be able to bring a lean condition back to normal, it can only do so up to a certain extent, thus the P0174 code is stored by the ECM/PCM to alert the user to resolve this issue.
More About P0174 and Lean / Rich Codes
The ECM/PCM and the O2 sensor are all about keeping things balanced in the combustion chamber.
If the mixture drifts too far out of balance, a rich or lean code is stored and ECM/PCM’s fuel trim strategy alters injector pulse width, moving the mix in the opposite direction of the condition to bring the air/fuel mix back into balance.
It should be noted that the oxygen sensor couldn’t care less about unburned fuel in the exhaust stream – the sensor is designed to sense oxygen, not fuel.
So, whenever any cylinder misfires due to spark or compression, there will be unburned fuel in the exhaust stream and the oxygen that didn’t unite with that fuel both passing the O2 sensor – but all the sensor will see is the oxygen.
In that case, a P0174 code might not indicate a lean condition, particularly if it is accompanied by misfire codes.
If the air/fuel ratio is off balance and there’s too much air, leftover oxygen molecules will enter the exhaust after combustion. This means that too much air and not enough fuel create a lean mixture.
Additionally, keep in mind that the O2 sensor cannot detect fuel and is unaware of it during a misfire when oxygen and fuel continuously leave a combustion chamber without any combustion occurring.
Potential Causes of a P0174 Code
The P0174 engine code signifies a problem with the air-fuel ratio. Unfortunately, many things can cause lean conditions, and it’s better to have a professional look at your vehicle to pinpoint the exact cause. While the error code does not show the cause of the issue, it offers a starting point for diagnosis.
Some potential causes of your engine’s lean condition include:
- An air leak after the mass airflow (MAF) sensor
- Clogged air filter
- Clogged fuel injectors
- Faulty fuel pump
- Faulty MAF sensor
- Clogged exhaust gas recirculation port
- Failing oxygen sensor
- Leaking brake booster
- Leaking positive crankcase ventilation (PCV) valve
- Incorrect fuel pressure
- Faulty coolant sensor
P0174 Code Symptoms
While driving with a P0174 code is safe, you may experience other symptoms if the problem persists.
Some other symptoms of issues that cause a P0174 code include:
- Engine power loss
- Rough or high idle
- Decreased acceleration
- Stalling
- Misfires and other codes
Vehicles in lean conditions emit nitrogen oxides (NOx), which harm human health and can cause respiratory issues like asthma. Running a car in lean condition causes a misfire, which harms the catalytic converter and ruins the vehicle’s internal parts in the long run.
If you notice the symptoms above, have your vehicle diagnosed by a professional to prevent further damage
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.
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.
How Much Does It Cost to Fix a P0174 Code?
A precision diagnosis tells you what repairs are needed to clear the P0174 code. A professional technician will diagnose the problem and give you an estimate for fixing it.
The rates will differ depending on your location, car model and the technician’s work. If you only need to clean the MAF, it can cost you as little as $100. Your repair costs may be as high as $1000 for vacuum leaks and $1300 to $1700 for fuel pumps.