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.
Detection Condition
DTC P0171 indicates air/fuel ratio is too lean (Bank 1).
DTC P0174 indicates air/fuel ratio is too lean (Bank 2).
Fuel control system uses fuel trim tables stored in PCM to compensate for variances in fuel system components from wear and aging. The fuel trim strategy learns deviations during closed-loop engine operation. Fuel trim has 2 means of adapting; a long term fuel trim and a short term fuel trim. Inputs from Engine Coolant Temperature (ECT), Cylinder Head Temperature (CHT) and Mass Airflow/Intake Air Temperature (MAF/IAT) sensors are required to activate fuel trim adjustments. The HO2S detects presence of oxygen in exhaust and provides PCM with feedback identifying air/fuel ratio. A fuel trim factor is added to fuel injector pulse width calculation according to long and short term fuel trims. Long term fuel trim values at their rich or lean limits indicate a fuel system malfunction.
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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.