If your Subaru Legacy is returning OBD II code P0171 when you scan for trouble codes, that's an indication of an overly-lean mix of gases coming through your exhaust system. In this article, we'll go over what that means, what causes it, and how to diagnose & repair the issues that might be causing it. This is a reasonably common code, so don't fret – there's plenty of information out there to help you get your Subaru Legacy back on the road.
What does OBD II code P0171 mean on a Subaru Legacy?
Diagnostic Trouble Code (DTC) P0171 indicates that an O2 sensor monitoring the exhaust flow associated with engine bank 1 has detected what's known as a "lean condition" – that is, there's too much oxygen in the exhaust. This is a common issue with many vehicles, and it's usually caused by a problem with the fuel system. As a reminder, engine bank 1 is the side of the engine with the #1 cylinder.
It's worth noting that codes P0171 and P0174 are very similar, and they're often caused by the same issues & are active at the same time. The difference between the two is that P0171 is associated with bank 1, while P0174 is associated with bank 2. If you're seeing code P0174, you can refer to the linked article for more context.
What triggers P0171?
This code is initially triggered by the initial downstream O2 (Oxygen) sensor, which is the first sensor after the exhaust leaves the cylinders. This sensor is responsible for keeping the fuel mixture in the engine at the correct ratio of fuel to air. If the sensor detects too much oxygen in the exhaust, it will trigger the code.
How does the downstream O2 sensor keep the fuel mixture correct?
The downstream O2 sensor is responsible for monitoring the exhaust flow, and it does this by measuring the amount of oxygen in the exhaust. The sensor sends this information to the engine's computer, which uses it to determine how much fuel to inject into the engine.
If the sensor detects too much oxygen in the exhaust, it will send a signal to the computer to reduce the amount of fuel injected into the engine. This will cause the engine to run leaner than it was. On the other hand, if the post-cylinder O2 sensor detects too little oxygen in the exhaust, it will send a signal to the computer to increase the amount of fuel injected into the engine. This will cause the engine to run richer than it had been.
The downstream O2 sensor is trying to maintain a target ratio of 14.7:1 – the ideal ratio of air to fuel for a gasoline engine.
P0171 Symptoms on the Subaru Legacy
There are a number of symptoms you can base your P0171 diagnosis on, but ultimately they're not incredibly noticeable. You'll usually notice this code from the check engine light & not the symptoms themselves.
That said, one of the big ones is reduced engine power from a lean fuel mixture. Sometimes you'll also see reduced fuel economy to match as your engine has to make up the difference.
How to diagnose & repair OBD II code P0171 on a Subaru Legacy
There are a number of considerations when diagnosing this code. We recommend going through the process as a series of tests, and checking off possible issues in order.
The first thing to check on? Check the Mass Air Flow (MAF) sensor. These sensors can get "oiled", which translates to "dirty". If the MAF sensor is dirty, it can cause the engine to run leaner than it should, which will trigger the code. Keep an eye out for "oiled" air filters in particular. Oiled air filters tend to release oily residue into the air intake, which can get into the MAF sensor and cause it to malfunction. Also check the silicone around the MAF for leaks – some vehicles have run into build quality issues here.
If your MAF sensor is dirty, you'll want to clean it. You can do this by spraying it with MAF sensor cleaner (which you can buy here), or (potentially less effectively) by using a can of compressed air to blow out the sensor. If you're using compressed air, make sure you're not blowing the sensor with too much force – you don't want to damage the sensor.
Check for vaccum leaks downstream of the MAF in the intake system. The MAF sensor won't be able to detect a leak in the intake system, so it will continue to report the same amount of air flowing into the engine. This will cause the engine to run leaner than it should, which will trigger the code.
Check the vaccuum lines & PCV (Positive Crankcase Ventilation) hoses for cracks or other damage. If you find any, you'll want to replace the relevant parts instead of trying to patch them.
Less Common Issues Associated With P0171
One of the issues you might think is more common than it actually is? A failed O2 sensor. These tend to be pretty bulletproof as sensors go, but they can fail. If you're seeing code P0171, you'll want to check the downstream O2 sensor for damage or other issues. If you find any, you'll want to replace it. We recommend buying parts on Amazon, as they're usually cheaper than buying them from a dealership & are almost as fast if not faster to arrive. You can purchase a Subaru Legacy O2 sensor here.
It's unlikely to be the issue, but you might also be running into issues with fuel supply. Check the fuel filter for clogs & make sure the pressure is within spec. If you're running into issues with fuel supply, you'll likely see other codes as well. Finally, you might also encounter an exhaust leak somewhere between your engine cylinders & the inital downstream O2 sensor that are throwing your readings off.
If all else fails, you'll need to get a solid diagnostic tool with support for fuel trim readings. This will allow you to see how much fuel is being injected into the engine, and how much is being burned. If you're seeing a lot of fuel being injected into the engine, but not a lot of it is being burned, you'll know that you're running into issues with fuel supply. If you're seeing a lot of fuel being burned, but not a lot of it is being injected, you'll know that you're running into issues with fuel delivery.
Conclusion & Our Usual Caveats:
OBD II Code P0171 is a Subaru Legacy code that indicates an issue with your exhaust air:fuel ratio. There are a number of possible underlying issues, each of which has multiple potential fixes. We hope this article has been helpful, but we'd like to remind you – Use the information above at your own risk. This article is not meant to be a substitute for professional advice. If you're not comfortable with the steps outlined above, we recommend taking your Subaru Legacy to a mechanic or dealership to have them diagnose & repair the issue.