Toyota P0171 – Meaning, Causes, Symptoms, & Fixes
Toyota Code P0171 Definition
System too lean (Bank 1).
What Does P0171 Mean For Your Toyota?
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 Toyota 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.
Toyota P0171 Symptoms
- Check Engine Light is on
- Lack of power from the engine
- Rough idle
- Engine coughing
- Engine misfiring
Toyota 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
Toyota Code P0171 Severity – Moderate
It is okay to drive your Toyota 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.
Toyota 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.
Toyota P0171 Diagnosis
- Tools Needed to Diagnose:
- Use FIXD to scan your Toyota to verify P0171 is the only code present. If other codes are present, they must be addressed first.
- 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.
- 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.
- 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.
- 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.