P0140 — O2 Sensor Circuit No Activity (Bank 1, Sensor 2)

P0140 means the downstream oxygen sensor signal is flat — the ECM sees no activity at all. An open circuit or a dead sensor are the two realistic causes.

Cost range: $120–$450. Severity: low. Safe to drive? Yes. It will block emissions readiness monitors from completing.

Most common causes of P0140

1. Open circuit in the sensor wiring — Check for a disconnected or melted connector first. ($50–$250)
2. Dead downstream O2 sensor — No output at any temperature. ($120–$400)
3. Blown sensor heater fuse — Often shared with other sensors — check for multiple O2 codes. ($5–$50)

Symptoms drivers report with P0140

• Check engine light
• Emissions monitors won't set
• No performance symptoms