Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Circuit Description
The DTC P0171 Fuel Trim System Lean diagnostic monitors Heated Oxygen Sensor (HO2S) indications of air/fuel mixture. To provide the best possible combination of driveability, fuel economy, and emission control, the Powertrain Control Module (PCM) uses a Closed Loop air/fuel metering system. PCM monitors HO2S signal voltage and when in closed loop adjusts fuel delivery based on HO2S signal voltage. Changes in fuel delivery will be indicated by long term and short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around zero percent. PCM will add fuel when heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above zero percent. PCM will reduce the amount of fuel delivered when a rich condition is indicated by HO2S. Fuel trim values below zero percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition. See FUEL TRIM SYSTEM OPERATION table.
| HO2 S Indication | Short Term FT Action | Long Term FT Action | Short Term FT Response |
|---|---|---|---|
| Lean Condition | Quick Increase of 1% Or More | Slow Increase Of 1% Or More | Return To Zero Percent |
| Desired Condition | Stays Near Zero Percent | Stays At Learned +/- Value | Stays Near Zero Percent |
| Rich Condition | Quick Decrease Of -1% Or Less | Slow Decrease Of -1% Or Less | Return To Zero Percent |
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.