Failed downstream oxygen sensor (bank 1 sensor 2) due to age, contamination, or internal circuit damage; Open or shorted wiring in the sensor signal circuit, including chafed harnesses or corroded connectors; Blown fuse or faulty relay supplying power to the oxygen sensor heater circuit; Exhaust leak before or near the sensor that lets outside air skew the reading; Contaminated sensor from coolant, oil, or silicone poisoning that coats the sensing element; PCM internal fault or corrupted software affecting the sensor signal processing (rare)

DTC codes / P01xx · Fuel and air metering / P0140

P0140O2 Sensor Circuit No Activity (Bank 1 Sensor 2)

Q: What does code P0140 mean?

O2 Sensor Circuit No Activity (Bank 1 Sensor 2). P0140 means the engine computer (PCM) is not seeing any meaningful voltage activity from the downstream oxygen sensor on bank 1, which is the sensor after the catalytic converter. The PCM expects the sensor's signal to switch within a normal range as it monitors oxygen storage in the converter, but the voltage has stayed flat or out of range long enough to set the code.

P0140 means the engine computer (PCM) is not seeing any meaningful voltage activity from the downstream oxygen sensor on bank 1, which is the sensor after the catalytic converter. The PCM expects the sensor's signal to switch within a normal range as it monitors oxygen storage in the converter, but the voltage has stayed flat or out of range long enough to set the code.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other symptoms besides the warning light
A slight drop in fuel economy because the PCM may default to a less efficient fuel strategy
Occasional rough idle or hesitation if the sensor's inactivity affects fuel trim calculations
Failed emissions inspection due to the oxygen sensor monitor not completing or reporting a fault
In rare cases, a sulfur or rotten-egg smell from the exhaust if the converter is being overworked

Common causes, most likely first

Failed downstream oxygen sensor (bank 1 sensor 2) due to age, contamination, or internal circuit damage
Open or shorted wiring in the sensor signal circuit, including chafed harnesses or corroded connectors
Blown fuse or faulty relay supplying power to the oxygen sensor heater circuit
Exhaust leak before or near the sensor that lets outside air skew the reading
Contaminated sensor from coolant, oil, or silicone poisoning that coats the sensing element
PCM internal fault or corrupted software affecting the sensor signal processing (rare)

How to diagnose and fix it

Scan all stored and pending codes, then clear them and road-test to see if P0140 returns before replacing parts
Inspect the sensor wiring and connector for damage, corrosion, or loose pins, and repair as needed
Check the oxygen sensor heater circuit fuse and relay, and verify power and ground at the sensor connector
Perform a voltage drop test on the signal circuit and compare sensor output to a known-good sensor or scan tool graph
Check for exhaust leaks upstream of the sensor using a smoke machine or by listening for ticking during cold start
Replace the downstream oxygen sensor if wiring and circuits test good but the signal remains inactive
After repair, clear codes and complete a drive cycle to verify the oxygen sensor monitor passes and the code does not reset

Why a downstream sensor goes inactive

A downstream oxygen sensor can fail without setting a heater code, so technicians who only check for P0135-style faults may miss a dead sensor signal circuit.
Exhaust leaks between the converter and the sensor are a common cause of a flat signal because outside air pulses past the sensor and the PCM interprets it as no activity.
Some vehicles use the downstream sensor to infer catalytic converter efficiency, so a P0140 can appear alongside P0420 even when the converter itself is still functional.
Clearing the code without repairing the root cause often leads to a repeat failure within a few drive cycles, especially if the sensor heater circuit is intermittent.
A sensor that reads a fixed rich or lean voltage can still set P0140 if the PCM sees no expected switching activity over the test period.

Can I keep driving with P0140?

You can usually drive the vehicle with P0140 set, but avoid long trips or heavy loads until it is diagnosed because the PCM may run a less efficient fuel strategy. Do not ignore the code indefinitely, as continued driving with a faulty downstream sensor can lead to catalytic converter damage and failed emissions tests.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.