P0171 and P0174: Meaning, Symptoms, Causes, and Fixes

P0171 and P0174 both mean “System Too Lean”. But what are the differences? What are the symptoms, Causes and How to Fix them? All are here in this article.

The P0171 OBD2 code can be difficult to diagnose the cause of. It generally tells you too much air is getting into your engine. While the P0171 code isn’t serious on its own, it can lead to further damage that is more serious. This makes it an important code to fix as soon as you see it.

Maintaining the right mix of air and fuel in your engine is important to maintaining peak performance. When the P0174 OBD2 code activates, you know this ratio is off. Too much air in your system can make you use more fuel and diminish your engine’s power.

The good news is, you can diagnose and often fix the P0171 and P0174 trouble code at home. Read on below to learn the steps you should take.

P0171 and P0174 Code Definition

P0171 Code Definition

P0171 Code (Generic): System Too Lean (Bank 1)

P0171 Chevy: Fuel Trim System Lean

P0171 Ford: System Too Lean (Bank 1)

P0171 Toyota: “System Too Lean (A/F Lean,Bank 1)”

P0174 Code Definition

P0174 Code (Generic): System Too Lean (Bank 2)

P0174 Ford: System Too Lean (Bank 2)

P0174 Toyota: “System Too Lean (A/F Lean, Bank2)”

P0174 Nissan: Fuel Sys Lean B2

P0174 BMW: “System Too Lean (A/F Lean, Bank2)”

What Do P0171 and P0174 Mean?

The code P0171 is telling you the air to fuel ratio in your engine is off. The ideal ratio for a combustion engine is 14.7:1 (14.7 parts air to each 1 part fuel). When the ratio of air to fuel is higher than this, it’s called a “lean” mixture.

Modern engines automatically compensate for minor issues with the ratio. For a lean mix, this means injecting more fuel to compensate. The trouble code P0171 is triggered when these adjustments get too large to maintain sustainably.

Code P0171 Diagnosis and Repair .

The trouble code P0174 means that the air to fuel ratio in your car’s engine is off. Specifically it’s saying the system is too lean, meaning there is too much air in the mix. It’s differentiated from the similar code P0171 because it points you to bank 2 of your system.

A ratio of 14.7 parts air to 1 part fuel is ideal for modern combustion engines. When the ratio of air to fuel is higher, the powertrain control module adjusts by injecting more fuel. P0174 triggers when these adjustments get too big and the system’s having trouble getting the correct ratio.

What Are The Symptoms of The P0171 and P0714 Code?

Symptoms of P0171 Code

The most common symptoms of a P0171 trouble code are:

  • Activation of the check engine light
  • Difficulty starting the engine
  • Engine stalling after warming up
  • Stumbling or hesitation from the engine
  • Rough idling
  • Reduced engine power
  • Misfires and engine coughs

These symptoms are also common to a variety of other engine issues. This makes it very difficult to diagnose a lean air to fuel ratio without use of a diagnostic scanner.

Symptoms of P0174 Code

You’ll notice a few drivability issues when P0174 triggers. The symptoms of this trouble code include:

  • Activation of the check engine light
  • Rough idling
  • Misfiring and coughing from the engine
  • Reduced power
  • Engine surges when accelerating
  • Reduced fuel efficiency

What Are The Causes of P0171 and P0174?

Causes of P0171 Code

The P0171 code is triggered when either the short-term fuel trim or the long-term fuel trim exceeds a certain value. This means the engine is adjusting the air to fuel ratio more than it should be.

Identifying the cause of the error code means first figuring out where the ratio is going wrong. Common underlying causes are:

  • A fault in the mass air flow sensor
  • A fault in the oxygen sensor
  • A fault in the fuel pressure regulator
  • Weakened or damaged fuel pump
  • Faulty or clogged fuel injectors
  • Faulty or clogged fuel filter
  • Vacuum leaks in hoses or gaskets
  • Leaks in the exhaust system

Causes of P0174 Code

There are a few different systems that can trigger the P0174 trouble code:

  • MAF (mass air flow) sensor. A dirty or faulty MAF sensor can make the engine think the mix is too lean.
  • Oxygen sensor. Similar to the MAF sensor, the oxygen sensor can malfunction, get dirty, or fail.
  • PCV (positive crankcase ventilation) line. Leaks in the PCV valve or damage and cracks on the vacuum connection are a common cause of P0174. The problem could also be that the PCV line itself is faulty or stuck open.
  • Vacuum leaks. Leaks in lines downstream of the MAF sensor are often the cause of P0174. It can also be triggered by cracks and damage in vacuum hoses, the PCV hose, or the intake manifold gasket.
  • Fuel pump. A weak or malfunctioning fuel pump can throw off the fuel pressure, which dirties the filters and leads to clogs.
  • Fuel injectors. A failed fuel injector will trigger the P0174 code. Clogged and dirty fuel injectors can also be the cause.
  • Exhaust system. The exhaust system could be leaking. A clog in the EGR (exhaust gas recirculation) port can also affect the air-to-fuel ratio.

How Serious Is The P0171 and P0174 Code?

A P0171 trouble code is not an emergency. You can continue driving your car while you wait to make the repair. Don’t put the repair off for long, though. In the long-term, a lean air to fuel ratio can cause your engine to overheat, leading to significant internal damage.

The trouble code P0174 does need to be addressed fairly quickly. Problems with the air-to-fuel ratio can overheat the engine, causing permanent damage. However, you can drive your car with the P0174 trouble code active for a short period.

How Do You Diagnose P0171 and P0174?

How to Diagnose P0174?

Tools you’ll need:

  1. Scan your car for other trouble codes. If any others come up, fix those first then scan again.
  2. Analyze both the STFT (short term fuel trim) and the LTFT (long term fuel trim) data for your vehicle. This shows you exactly how your system has been compensating for the air-to-fuel ratio and can help you identify the origin of the problem.
  3. Inspect the vacuum lines and hoses in your engine for leaks. The easiest way to find a vacuum leak is to listen for a hissing sound while they engine’s running. You can also look for cracks or tears and feel the tubes for spots that are hard or warped. Replace any damaged tubes. While you’re at it, check all the connections to make sure they’re tight.
  4. Remove the MAF sensor and clean it with contact cleaner or brake cleaner. After you’ve reinstalled the sensor, clear the trouble codes and scan your system again. If P0174 is still present, continue with step 4.
  5. Check for an exhaust leak by starting the engine and having an assistant idle it at 1,500rpm. Listen to the engine for exhaust noises in the manifolds or head pipes. Seeing black smoke at these points can also indicate a leak.
  6. Use a fuel pressure gauge to test your system’s fuel pressure. Once you have the reading, check your car’s specifications to find out what your fuel pressure should be. If the reading is too low, check if the filter or line are clogged. You can also check the voltage of the pump with a multimeter to make sure it’s still functioning correctly.

If none of the above steps identifies the problem, test the oxygen sensor using a multimeter. Visually inspect it for dirt and clean with contact cleaner, then re-install and scan again.

How to Diagnose P0171?

As you can see above, there are a lot of systems that could trigger this trouble code. The good news is there are some simple steps you can follow to pinpoint the exact source of the problem.

  1. Scan for other trouble codes. If you find any, fix those first.
  2. Check for vacuum leaks in the engine by using a vacuum gauge. Do this with the engine turned off so you can hear the air escaping through any leaks that are present.
  3. Fix all causes of vacuum leaks, reconnect all components, and clear the trouble codes. Scan your system again. If the trouble code still comes up, move on to step 4.
  4. Take out the mass air flow sensor and clean it with contact cleaner. Reinstall it, clear the codes, and scan your system. If the code still comes up, move on to step 5.
  5. Check your fuel filter and fuel pump. Remove and clean them, then re-install all components and use a fuel pressure gauge to test the fuel pressure sensor and mass air flow sensor.
  6. Start your engine and get it to operating temperature. Check the Barometric Pressure reading from the mass air flow sensor. If it’s reading around 26 Hg, the mass air flow sensor is likely malfunctioning and under-reporting the air density.
  7. If no issues are found with the tests above, test your oxygen sensors. They likely need to be replaced.

What Common Mistakes Should You Avoid While Diagnosing Code P0171 and P0174?

The biggest mistake most people make is not following through on the complete diagnosis. They might replace the air fuel sensor when the real problem is further down the line.

The P0171 code is also often misdiagnosed as an issue with the oxygen sensor. Pay attention to the freeze frame readings to be sure you’re replacing the right component.

For P0174, the most frequent error people make is not completing an entire diagnostic. Many times, when people get a bad reading they replace the oxygen or MAF sensor before checking further. This won’t solve the problem if there’s a clog or failure elsewhere in the system. Make sure to fully analyze the fuel trims and freeze frame data before replacing any components.

What Should You Do to Fix P0171 and P0174 Codes?

You can detect the p0171 and P0174 codes by using an OBD2 scanner.
OBD2 scanners can help you detect the p0171 code.

How to fix P0171?

Start by repairing any vacuum leaks, as suggested in step 3 of the diagnosis above. Not only is this a common cause of issues with the air to fuel ratio, it’s also the cheapest and easiest fix.

If the vacuum seals are intact and the error still comes up, go through the following steps:

  1. Remove the mass air flow sensor and clean it with brake cleaner or electronics cleaner. Make sure it’s completely dry before reinstalling it.
  2. Check the fuel filter. If it’s dirty or not maintaining proper fuel pressure, replace it.
  3. Use an advanced scan tool to read the short-term fuel trim and the long-term fuel trim. This will show you exactly how much the powertrain control module is adjusting the air/fuel mixer. You can use this data to tell you if you need to replace the fuel pressure regulator, fuel pump, fuel injector(s), oxygen sensor, or the entire powertrain control module.

How to Fix P0174?

Cleaning the MAF filter and repairing vacuum leaks usually clears the P0174 code. If they don’t, you’ll likely need to replace one or more components, following these steps:

  1. Repair any leaks found in the exhaust system and replace any dirty filters or clogged lines.
  2. Check the fuel pressure of your system again. If it’s still too low, test the fuel pump and fuel injectors and replace as needed.
  3. Test the oxygen sensor and the MAF sensor. Replace them if they’re dirty or damaged.
  4. Test the PCV valve and EGR valve, replacing as needed.

After every step, use an OBD2 scanner to clear the codes and check your system to see if the issue has been resolved.

Tips To Avoid P0171 and P0174 In The Future

How to Avoid P0171?

Preventing extra air from getting into your engine will help you maintain the right air to fuel ratio long term. Checking for leaks in the vacuum lines and gaskets is quick and easy:

  1. Open the hood and visually inspect all lines for cracks and damage. These will most often be found on the ends of the line.
  2. Turn on your engine. While your car is idling, spray carburetor cleaner around the gaskets. If the engine speed increases, you have a leak.

An exhaust leak can also let air into your fuel system, eventually leading to a P0171 code. To check for an exhaust leak:

  1. Start your car and have an assistant idle at about 1500rpm.
  2. Stand by the engine and listen. If exhaust noises are coming from the head pipes or manifolds, you likely have a leak.
  3. Look for black soot coming out anywhere in the system. This is another clear indication of an exhaust leak.

How to Avoid P0174?

Over-oiling engine components like the MAF sensor can cause them to attract dirt, which can stick and damage or clog the system. Check and clean your engine regularly to prevent clogs and debris build-up, common causes of damage to lines and components.

If you need to replace components, do your best to identify why they failed. The oxygen sensors, for example, are often damaged by leaking coolant. If you don’t locate and fix the source of the leak, your new sensors will fail just like your last ones.

Read more: OBD2 Codes: Full List Meaning & Fix Guide

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Diagnostic Tools and Auto Repair". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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