P0171
Fuel Trim Malfunction (Bank 1)
Powertrain Fuel and Air Metering Fuel Trim Control 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Check Engine Light illuminated
Rough idle or hesitation during acceleration
Poor fuel economy
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Air filter
Replace a clogged air filter to restore proper airflow and sensor readings.
2
Oxygen sensor (Bank 1)
Remove and replace the upstream O2 sensor if it's slow to respond or reading incorrectly.
3
Fuel injectors or fuel pump
Clean or replace fuel injectors, or test fuel pressure regulator if injectors are not delivering adequate fuel.