OBD2 Codes

Honda P0171 – Meaning, Causes, Symptoms, & Fixes

P0171 system too lean bank 1
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Honda Code P0171 Definition

System too lean (Bank 1).

What Does P0171 Mean For Your Honda?

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 your Honda 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.

Honda P0171 Symptoms

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

Honda P0171 Causes

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

Honda Code P0171 Severity – Moderate


It is okay to drive your Honda 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.

Honda 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 air flow 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.

Honda P0171 Diagnosis

  1. Use FIXD to scan your Honda 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.
    • 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.
  3. Remove the mass air flow sensor and clean the sensor using mass air flow cleaner or contact cleaner.
    • Reinstall the mass air flow sensor and clear the check engine light using FIXD. If the check engine light comes back on with code P0171 continue the diagnostic process.
  4. If check engine light code P0171 persists after you have reconnected and replaced all damaged vacuum lines and cleaned the mass air flow sensor, perform a fuel pressure test. If any components in the fuel system are failing, replace them as necessary.
  5. If you confirm there are no vacuum leaks, have a clean mass air flow sensor, and your fuel system is functioning properly, it is time to replace the oxygen sensors.



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  1. Thanks! Pretty much good information!

  2. Thanks I made the mistake of jumping straight to the Oxygen sensors, didn’t check other things mentioned , let me give those a try

    1. Similar situation for me… How has it gone for you so far?

  3. Did everything that was suggested and finally bit the bullet and got an upstream 02 sensor and bought the 02 sensor socket 30.00 bucks for it and 248.00 for upstream 02 .when I went to take the old upstream 02 sensor out I found out it was barely screwed in ….so I took it out cleaned it with a wire brush and a can of parts cleaner tigghtend it down and reset check engine light ..ran it down the road about 20 mile at 50 mph ..check engine stayed off …great news huh ..would have saved me almost 300.00 bucks if I hadn’t opened the new upstream 02 sensor because my parts store will not take them back if you open the pack .even though I didn’t even screw it in or plug it up but at least I have a spare now or if someone I know really needs one.

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