P0131 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)

P0131 means the upstream oxygen sensor on bank 1 is reading persistently low voltage — the ECM sees it as a permanently lean exhaust. Check for exhaust leaks before replacing the sensor.

Cost range: $50–$500. Severity: medium. Safe to drive? Yes short term. Fuel economy suffers and the catalytic converter can be affected over time.

Most common causes of P0131

1. Exhaust leak ahead of the sensor — Fresh air pulled in reads as lean. Cheaper to weld than to replace parts you don't need. ($100–$400)
2. Failed upstream O2 sensor — Most common after 100k miles. ($150–$400)
3. Wiring short to ground — The sensor harness runs near hot exhaust and melts. ($80–$250)
4. Genuinely lean condition — Look for P0171 alongside — then fix the leak, not the sensor. ($50–$500)

Symptoms drivers report with P0131

• Poor fuel economy
• Rough idle
• Check engine light
• Failed emissions test