P0140: O₂ Sensor Circuit – Bank 1 Sensor 2 – No Activity Detected

This code means the downstream O₂ sensor (after catalytic converter) is not switching voltage. The ECU sees a flat line (no change) for a long time.

✅ P0140 – Common Causes

1. Faulty O₂ Sensor (Bank 1 Sensor 2)

Sensor heater may work, but signal element is dead.

Sensor output stuck around 0.45 V (flat line).

2. Wiring Issues

Broken signal wire.

Short to ground or short to power.

Corroded connector (very common near exhaust).

3. Exhaust Leak Before B1S2 Sensor

Extra oxygen can cause the sensor to show no switching.

4. Bad ECU Ground / Reference Voltage

Rare, but possible.

5. Contaminated Sensor

Oil/antifreeze contamination from engine issues.

✅ How to Diagnose (For Workshop Use)

1. Scan Live Data

Check B1S2 sensor voltage:

Normal: should fluctuate slowly between 0.1–0.9 V during closed loop.

Faulty: stays fixed around 0.45 V or 0.0 / 1.0 V.

2. Check Heater Operation

Even though it’s not a heater code, test:

Heater resistance: 3–15 ohms typically

12 V supply to heater present?

3. Visual Inspection

Check wiring harness near exhaust (burnt, melted).

Clean and re-seat connectors.

Check for rust on connector pins.

4. Check Signal Wire With Multimeter

Back-probe the O₂ signal wire:

Rev engine to 2500 rpm.

Voltage should change.

If flat → sensor faulty or wire broken.

5. Check for Exhaust Leak

Look at:

Flex pipe

Catalytic converter flange

Gasket before B1S2

Even a small leak can cause no switching.

✅ Most Common Fixes

🎀 Replace O₂ Sensor (Bank1 Sensor2)

Most cases (70–80%) are solved by replacing the downstream sensor.

🎀 Repair Wiring

If:

Signal wire open

Connector corroded

Wire burnt by exhaust

🎀 Fix Exhaust Leak

If leak is before the sensor.

🎀 Clear Code and Test Drive

Check O₂ live data again after repairs.

Welcome To Madurai Car Service 💐🙏🏻.