P0171 – Meaning, Causes, Symptoms, & Fixes

Code P0171 Definition

Bank 1 has too much air or not enough fuel.

What Does P0171 Mean?

Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. When there are more than 14.7 parts air to 1 part fuel in the air-fuel mixture, a lean condition exists and code P0171 is triggered. The lean condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the lean condition by injecting more fuel to the mixture in an effort to maintain the proper 14.7:1 ratio. When these adjustments become too large, code P0171 is triggered.

What Are The Symptoms Of P0171?

• Check Engine Light is on
• Lack of power from the engine
• Rough idle
• Engine coughing
• Engine misfiring

What Is The Cause Of P0171?

• Dirty or faulty mass airflow sensor
• Vacuum leaks – PCV hoses, vacuum hoses, intake manifold gasket.
• Weak fuel pump
• Clogged or dirty fuel injectors
• Clogged fuel filter
• Exhaust leak
• Faulty oxygen sensor
• Faulty air-fuel ratio sensor

How Serious Is Code P0171? – Moderate

It is okay to drive a vehicle with P0171 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.

Code P0171 Common Diagnosis Mistakes

It is important to complete the entire diagnostic process when diagnosing P0171. Many people will replace the air-fuel sensor or O2 sensor as soon as they get a bad reading, but the root cause is often a dirty or faulty mass airflow sensor or vacuum leak, thus causing the O2 or A/F sensor to read differently to compensate. Reading and analyzing fuel trims and the freeze frame data is the key to properly diagnosing P0171.

Tools Needed To Diagnosis P0171

• Tools You May Already Have:
  • Soapy water
  • Screwdriver
  • Pliers
• Tools You May Need (FIXD’s top recommended picks from Amazon):
  • FIXD ($59.99)
  • Mass Air Flow Cleaner ($11.31)
  • Fuel Pressure Testing Gauge ($25.99)
  • Hayne’s Repair Manual ($19-$25)

How To Diagnosis Code P0171

Difficulty of Diagnosis and Repair – 3 out of 5

1. Use FIXD to scan your vehicle to verify P0171 is the only code present. If other codes are present, they must be addressed first.
2. Inspect all vacuum lines and hoses for leaks, and make sure they are properly connected. If a leak is present, you will hear a hissing sound, though it may be difficult to hear with the engine running. If you suspect a vacuum leak but are having trouble finding it, check out this detailed guide to finding vacuum leaks.
3. Reconnect any disconnected vacuum lines and replace any damaged vacuum lines that are leaking, then use FIXD to clear check engine light code P0171. If check engine light code P0171 is triggered again, check the rest of the vacuum lines for leaks and continue to the rest of the diagnostic process.
4. Remove the mass airflow sensor and clean the sensor using mass air flow cleaner or contact cleaner.
5. Reinstall the mass airflow sensor and clear the check engine light using FIXD. If the check engine light comes back on with code P0171 continue the diagnostic process.
6. If you confirm there are no vacuum leaks, have a clean mass airflow sensor, and your fuel system is functioning properly, check the exhaust for leaks before the Air Fuel Ratio Sensors or Oxygen sensors if you find any leaks have them repaired.
7. At this point if the code still persist you may want to consider replacing your A/F sensor or O2 sensors.

Estimated Cost of Repair

For error code P0171, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

• Vacuum leak $100-$200
• Clean MAF $100
• Replace MAF $300
• Fuel Pump $1300-$1700
• Fuel pressure regulator $200-$400
• Exhaust repair $100-$200 (if welded to repair)
• Air fuel sensor or oxygen sensor $200-$300

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