Summary
P0131 means the upstream O2 sensor on Bank 1 is reporting a voltage below 0.1V for an extended period, indicating a persistent lean signal. This can mean the sensor itself is failed (40%), the engine is actually running lean due to a vacuum leak or fuel issue (35%), or there's an exhaust leak introducing ambient air near the sensor (15%). Check whether P0171 (lean Bank 1) is also present — if so, fix the lean condition first.
Severity: Moderate
Safe to drive: Yes, short distances
Repair cost: $25–$300 depending on cause
DIY difficulty: Easy to Moderate
What does P0131 mean?
The upstream O2 sensor produces a voltage between 0V and 1V based on exhaust oxygen content. Lean exhaust (excess oxygen) produces low voltage (under 0.45V), rich exhaust (low oxygen) produces high voltage (above 0.45V). Normal operation shows rapid oscillation between ~0.1V and ~0.9V.
P0131 sets when the sensor stays below its low threshold (typically 0.1V) for too long. This means either the exhaust really is lean (and the sensor is accurately reporting it) or the sensor is stuck/biased low and misreporting.
The key diagnostic question: is P0171 (lean) also present? If yes, the engine is actually lean and the sensor is working — fix the lean condition. If P0131 is alone, the sensor or its circuit is likely at fault.