Fuel Trim Malfunction (Bank 1)
Your engine's computer can't balance the fuel mixture on Bank 1 (the side with cylinder #1) because a sensor is giving bad data or a fuel system part is failing. It's like trying to follow a recipe when your measuring cup is broken—the proportions are always wrong.
The ECM monitors oxygen sensor readings and adjusts fuel injector pulse width via short-term fuel trim to maintain a 14.7:1 air-fuel ratio. When trim corrections exceed ±25%, the ECU sets P0171, indicating the system cannot compensate for a lean condition despite maximum correction attempts.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Short-Term Fuel Trim (STFT) | -10% to +10% | >+25% (lean condition) |
| O2 Sensor Voltage (Bank 1) | 0.4–0.6V (switching) | Stuck high >0.7V or unresponsive |
Code P0171 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0171 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.