Can you keep driving?
Can you keep driving?
Stop driving if any of these apply
- !The engine is misfiring, stalling, or the check-engine light starts flashing.
- !There is a strong exhaust smell or a drivability change that suggests a bigger fault than the sensor alone.
What to check first
Step-by-step checks
- 1
Free - no tools
Let the exhaust cool fully before touching the rear sensor or nearby wiring
- 2
Free - no tools
Inspect the connector and harness for heat damage, corrosion, or a loose fit
- 3
Basic tool needed
Check for exhaust leaks around the sensor bung or downstream pipe
- 4
Basic tool needed
If scan data is available, compare the downstream sensor pattern with the upstream sensor before buying parts
- 5
Basic tool needed
If the engine is running badly, fix that first before assuming the sensor alone is dead
If the code returns
- -If the exhaust is sealed and the signal is still flat, the sensor or wiring becomes more likely.
- -If the code returns after a replacement, re-check the connector and exhaust side first.
- -If the signal wakes up after a wiring repair, that is a stronger clue than the code name alone.
Background
What this code means
P0140 is a generic OBD-II oxygen-sensor code for bank 1 sensor 2.
This is the downstream sensor on bank 1, so the fault can come from the sensor itself, its wiring, or an exhaust issue near that sensor.
The vehicle may still run normally, but catalyst monitoring can become unreliable if the signal stays flat.
Diagnosis
Common causes
Failed downstream O2 sensor
The sensor can stop switching even if the engine still runs.
Connector or harness issue
Heat damage near the exhaust can interrupt the signal.
Exhaust leak near the sensor
Fresh air around the sensor can flatten the reading.
Catalyst-related behavior
The catalyst can make the downstream pattern look abnormal.
Avoid these mistakes
What not to do
- xDo not replace oxygen sensors first if there is an obvious exhaust leak or mixture problem.
- xDo not ignore rough running just because the code names a sensor.
Parts
Parts that may need replacing
See also
Related OBD codes
Source notes
Generic OBD-II (SAE J1979 / ISO 15031-5). P0140 was expanded around common bank 1 sensor 2 no-activity faults, including sensor failure, exhaust leaks, and wiring issues.
This guide is written as a generic multi-make reference, so bulletin history, sensor locations, and repair order can still change by manufacturer and engine family.
This is generic OBD-II guidance and should not override vehicle-specific service information. Exact diagnosis and repair steps vary by make, engine family, and model year.
Last reviewed: 2026-04-10
Reference: Open reference