Introduction
The P0140 diagnostic trouble code indicates that the downstream oxygen sensor (Bank 1, Sensor 2) is showing no voltage activity or signal output. The PCM expects a stable, somewhat flat signal from this post-catalyst sensor; complete signal loss triggers code P0140.
Common Symptoms
- Check Engine Light illuminated on the dashboard.
- Failure during state smog or emissions inspections.
- Minor decrease in fuel efficiency (in some vehicles).
Primary Causes
- Dead Oxygen Sensor: Internal failure or complete circuit break inside Sensor 2.
- Disconnected Wiring Harness: Unplugged or broken wiring harness leading to the downstream sensor.
- Blown Sensor Fuse: Loss of 12V power supply to the heater or sensor circuit.
- Exhaust System Damage: Severed wires melted against hot exhaust pipes.
Step-by-Step Diagnosis & Fixes
- Inspect Sensor Plug: Verify that the downstream O2 sensor connector is securely locked and free of corrosion.
- Check Fuses: Inspect vehicle fuses related to sensor circuits.
- Test Sensor Voltage Output: Monitor live data on a scanner; a dead sensor will output a flat 0.45V or 0.0V without fluctuation.
- Replace Downstream Sensor: Replace Bank 1 Sensor 2 if wiring and power supply check out but the sensor produces no signal activity.