Failed downstream O2 sensor (Bank 1, Sensor 2) (50% likelihood): The oxygen sensor has completely failed or its sensing element has become contaminated and is no longer producing a varying voltage signal in response to exhaust gases. Open or shorted wiring in the O2 sensor circuit (25% likelihood): A broken wire, corroded connector, or short circuit in the sensor's signal wiring prevents the voltage signal from reaching the PCM. Blown fuse in the O2 sensor heater circuit (10% likelihood): If the sensor heater fuse is blown, the sensor may not reach operating temperature and will appear inactive to the PCM. Exhaust leak upstream of the sensor (10% likelihood): A significant exhaust leak before the sensor can dilute exhaust gases enough that the sensor sees a nearly constant oxygen level and does not switch. PCM issue or ground fault (5% likelihood): In rare cases, a poor ground connection or PCM fault can prevent proper reading of the sensor signal even though the sensor itself is functional.

How much does it cost to fix P0140?

DIY repair typically costs $30–$150 for parts. Professional repair runs $150–$400 including parts and labor.

P0140

Powertrain

Low

O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)

Get Your FREE AI Diagnostic Report

Enter your vehicle for a personalized diagnosis for code P0140

The P0140 code means the PCM has detected no switching activity from the downstream oxygen sensor on Bank 1. Normally this sensor should fluctuate its voltage output as it reads varying levels of oxygen in the exhaust gas after the catalytic converter. When the signal flatlines — staying at a constant voltage or showing no variation — the PCM determines the sensor circuit has no activity and triggers this code.

For most drivers, this code won't cause immediately noticeable drivability problems because the downstream sensor's primary role is monitoring catalytic converter efficiency rather than controlling the air-fuel mixture. However, the check engine light will remain on and the vehicle will fail emissions testing until the issue is resolved.

The most likely fix is replacing the O2 sensor itself, which is a common wear item. Before replacing it, inspect the electrical connector and wiring harness for corrosion, breaks, or heat damage. Check the sensor heater fuse as well, since the sensor needs to reach operating temperature to function. This is generally a beginner-friendly DIY repair requiring only a wrench or O2 sensor socket.

Severity

Low — MonitorThis Month

Symptoms

•Check engine light is on
•Reduced fuel economy
•Vehicle fails emissions inspection
•Slight rough idle in some cases
•No noticeable change in engine performance for most drivers
•Exhaust odor may be slightly stronger than usual

Likely Causes

Failed downstream O2 sensor (Bank 1, Sensor 2)50%

The oxygen sensor has completely failed or its sensing element has become contaminated and is no longer producing a varying voltage signal in response to exhaust gases.

Open or shorted wiring in the O2 sensor circuit25%

A broken wire, corroded connector, or short circuit in the sensor's signal wiring prevents the voltage signal from reaching the PCM.

Blown fuse in the O2 sensor heater circuit10%

If the sensor heater fuse is blown, the sensor may not reach operating temperature and will appear inactive to the PCM.

Exhaust leak upstream of the sensor10%

A significant exhaust leak before the sensor can dilute exhaust gases enough that the sensor sees a nearly constant oxygen level and does not switch.

PCM issue or ground fault5%

In rare cases, a poor ground connection or PCM fault can prevent proper reading of the sensor signal even though the sensor itself is functional.