BMW P0171 & P0174 Codes - Symptoms, Repairs, DiagnosticsPin

BMW P0171 Fault Code

Jake Mayock

Meet Jake

Jake currently owns two N54 powered BMW’s – an E60 535i and E82 135i. Jake has 10 years of experience maintaining, repairing, and upgrading his BMW’s. The 135i features a single turbo Precision 6266 conversion capable of 700+whp; Jake completed the entire project on his own. With over 200 automotive articles published, Jake brings a balance of writing skill, hands-on BMW experience, and technical knowledge to the table.

Engine fault codes are also known as a diagnostic trouble code – or DTC for short. These codes help point you in the direction of any potential issues with the engine. If you own a car for the long-run then chances are you’ll run into at least a few fault codes along the way. BMW P0171 is a common trouble code on a number of BMW engines.

What does code P0171 mean? What are the common symptoms, causes, and repairs? In this article, we discuss symptoms, causes, fixes, severity, and more regarding BMW P0171 fault code.

BMW P0171 & P0174 Codes - Symptoms, Repairs, DiagnosticsPin

BMW P0171 DTC Meaning

P0171 – System Too Lean (Bank 1)

Code P0171 indicates the engines computer (commonly known as the DME) thinks the engine is running too lean on bank 1. On inline-6 BMW engines, bank 1 refers to cylinders 1-3. Lean refers to the air-fuel ratio (AFR) and means there is too much air and too little fuel in the system.

However, this code rarely means the engine is actually running lean. Instead, the DME is perceiving a lean condition and injecting more fuel to compensate. This can mean your BMW engine is actually running rich and there’s too much fuel for the given amount of air. More on this as we move through BMW P0171 fault code.

Common Codes With P0171

P0174 – System Too Lean (Bank 2)

Fault codes P0171 and P0174 are often found together. The codes are the same and simply indicate bank 1 vs bank 2, respectively. The cause is often different if you only have one of these codes versus both of them at the same time.

On the popular BMW inline-6 there is really only 1 bank on the engine since the cylinders are inline. However, two banks exist on the exhaust side of the engine. Even though it’s a single exhaust manifold it’s split into two banks with an oxygen (O2) sensor for each bank. This is done to help narrow down issues when troubleshooting.

Anyway, we will circle back to the P0174 fault code several times in this article. It’s just important to understand the above notes to help diagnose the cause of BMW P0171 and/or P0174.

P0171 Fault Code Symptoms

Common symptoms of DTC P0171 include:

  • Check engine light
  • Power loss
  • Rough idle
  • Misfires
  • Slow start / long crank

A check engine light is the most common symptom of this fault code. Sometimes it may even be the only symptom. Depending on the underlying issue you might notice power loss, rough idle, misfires, or long cranking. Symptoms can be more serious with certain less common causes of BMW P0171 or P0174 codes.

Is BMW P0171 Code Serious?

Severity Rating: Moderate

Generally, DTC P0171 is not a serious problem. However, some of the symptoms can impact performance and drivability so use your judgement and err on the side of caution. If your BMW feels extremely sluggish or something seems way off then play it safe and have the car towed. Regardless, it’s still important to address the issue as soon as possible.

If your BMW isn’t actually running lean then the DME is injecting additional fuel which leads to a rich air-fuel ratio. This can put extra wear on the catalytic converter, which is not a cheap replacement. On the other hand, if the engine is running lean that can potentially cause severe engine damage. Fuel helps in cooling the combustion chamber, so too little fuel can lead to overheating, piston & valve damage, and other serious issues.

In summary, it’s usually safe to continue driving for short periods with BMW P0171 code present. Leaving the problem for too long may cause other engine problems, so it’s best to address as soon as possible.

Causes of P0171 BMW Fault Codes

Some of the most frequent causes of only BMW P0171 code include:

  • Faulty O2 sensor
  • Faulty fuel injector(s)
  • Intake leak near cylinder head

Notice these are issues that affect a specific engine bank. For example, a faulty cylinder #1 fuel injector would only affect the AFR on bank 1 and should not trigger the P0174 bank 2 code. The same can be said for an O2 sensor issue since there are different sensors for each bank. An intake leak could also cause BMW P0171, but it would have to be close to the cylinder head.

If you have both P0171 and P0174 codes then the common causes may include:

  • Intake leak before manifold
  • Low fuel pressure (fuel pump, rail, hoses etc.)
  • Positive Crankcase Ventilation (PCV) system leak
    • Including valve cover & gasket leaks
  • Mass Air-Flow (MAF) sensor
  • Vacuum leaks

With both P0171 and P0174 present the common causes are related to issues that can affect both engine banks. For example, the fuel pump feeds all 6 injectors so a fuel pump problem would cause issues on both engine banks.

Diagnosing P0171 DTC

Often, P0171 and P0174 present themselves together and the PCV (also called CCV) system and vacuum leaks are the two most common issues. If you only receive P0171 then the O2 sensor is the usual culprit. It’s important to actually do some diagnosing to find the issue, though. You likely don’t want to spend a bunch of money only to learn you didn’t fix the actual problem. Here’s a quick list of steps to help diagnose BMW P0171 fault code:

  1. Use an OBD scanner to confirm if any other codes are present. Clear the fault codes and drive for ~15 minutes (if safe to drive) to see if they return. If the codes return, it’s time to diagnose the issue.
  2. If the code returns then start with the basics. Are there any visible leaks? Are vacuum lines disconnected or visibly damaged? Does the O2 sensor wiring look OK?
  3. Conduct smoke test. This will point out any potential vacuum/intake leaks.
  4. Test O2 sensors, fuel injectors, fuel pump, and MAF sensor
  5. Once you believe you’ve traced and fixed the issue then delete the stored codes, drive for 15 minutes, and recheck to confirm the issue is resolved

BMW P0171 Fix & Repairs

Always remember to double check any codes stored in the DME. Sometimes fuel pump, injector, and other problems will throw another code. The above diagnostics are a good order for diagnosing P0171, but there might be a better starting point. For example, if you receive fueling codes alongside BMW P0171 code then starting with fuel system diagnostics likely makes more sense.

However, P0171 and P0174 are usually due to vacuum or PCV related issues that can be detected with a smoke test. Always start with the basics, though. Sometimes the problem is as simple as a hose or wire that popped loose.

We don’t always recommend replacing things that haven’t been diagnosed. However, you may consider replacing inexpensive parts such as vacuum hoses. We do always believe it’s a good idea to replace old, inexpensive parts on aging BMW’s while you’re in the area. It can go a long way in preventing future headaches.

P0171 BMW Fault Code Repair Cost

Now, hopefully you were able to track down the root cause of your BMW P0171 code issues. But what’s it going to cost? How expensive are the common causes of DTC P0171? Below is a breakdown of rough costs for the common causes:

  • Vacuum lines: $5-20
  • PCV/Crank vent hose: $25-100
  • O2 sensor: $75-200
  • MAF sensor: $75-200
  • Valve cover gasket: $20-50
  • Fuel injector: $50-300 (per injector)

Exact cost of parts can vary a lot since P0171 and P0174 codes may present themselves on any year or model BMW. Ultimately, the cost of fixing these fault codes isn’t too high. Costs can add up if you end up at a repair shop for diagnostics and repairs, though.

BMW P0171 FAQ’s

To close out the article we are simply listing some frequently asked questions about BMW P0171 fault codes. Please note – much of this info was already covered in the bulk of the article above, so we’re just consolidating some info here.

What Does DTC P0171 Mean?

Code P0171 means: System Too Lean (Bank 1). It’s commonly found alongside P0174 which indicates System Too Lean (Bank 2). These codes pop up when the DME believes the air-fuel ratio is too lean and has too much air and/or too little fuel.

How Expensive Are BMW P0171 Repairs?

Vacuum lines are one of the most common causes of P0171 and P0174. These lines are very inexpensive, so the repair can often be done for less than $20. Other common issues like a fuel injector, O2 sensor, or MAF sensor may run about $50-300.

Can You Drive With Code P0171 Present?

Continuing to drive with BMW P0171 and P0174 codes is generally safe. Use your own judgement and err on the side of caution; don’t continue driving if the vehicle feels unsafe or something seems severely wrong. You should still diagnose and repair the problem as soon as possible. Certain causes of DTC P0171 may cause additional problems if left too long.

For more BMW fault codes, check out our N54 common fault code guide and P1345 DTC guide.

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