Error P0171: Fuel-Air Mixture Too Lean

The P0171 error code indicates that the fuel-air mixture in your Honda Accord (8th Generation) is too lean, meaning there’s too much air in the mix and not enough fuel. This issue can affect vehicle performance and emissions, and addressing it promptly is important to maintain engine health.

The absence of accompanying error codes focuses the troubleshooting on fuel mixture issues directly. According to Honda’s troubleshooting guidelines for DTCs P0171 and P0172, and considering the conditions under which the measurements were taken (coolant temperature ~90°C, crankshaft RPM ~650, and transmission in Neutral), several factors were reviewed:

Fuel Pressure: The fuel pressure was within the normal range of 330-380 kPa (3.4-3.9 kgf/cm²), suggesting that the fuel pump and regulator are functioning correctly.
Air Flow Meter: The Mass Air Flow (MAF) sensor readings were initially low (1.2-1.6 g/s) but normalized (2.1-2.4 g/s) after addressing the issue, indicating that the sensor is working correctly but was affected by the lean mixture.
Throttle Body: While the throttle body was slightly dirty, it was not the root cause of the lean mixture. Regular cleaning of the throttle body can ensure optimal air intake and engine performance.
Air Leak: A damaged EVAP canister purge valve hose was identified as the source of a vacuum leak, allowing excess air into the combustion chamber and leaning out the fuel mixture.
Vacuum leaks are a common cause of P0171 errors and can often be found in aged or damaged hoses and seals.

Corrections Made:

Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values normalized significantly from 46.9% and 24.2% to 0.8%, indicating that the correction to the air leak restored the proper fuel-air ratio.
MAF sensor readings increased, reflecting a corrected air intake volume/
Oxygen sensor voltages adjusted to normal operating ranges, indicating improved combustion efficiency from 0.05-0.065 V to 0.60-0.68 V

Subjective Changes Noted:

The “kick” felt during smooth braking before a complete stop was eliminated.
The deceleration jerk experienced when completely releasing the gas at low speeds was resolved.
The persistent gasoline smell in the exhaust during cold starts was no longer present.

Total cost: 39 USD
Mileage: 201 900 km