What P0140 means
The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim.
How the engine computer detects it
P0140 means the sensor produces no voltage of its own and the control module only sees its own reference (bias) voltage. GM's V6 sets it when the signal sits at 401–548 mV or the sensor's internal resistance exceeds 40 kΩ when it should be hot; newer GM systems apply 1900 mV and read above 1700 mV as an open circuit. Two trips. Ford reports 'no activity' under P0136 instead. A cold sensor with a failed heater can also cause it. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- Live data: sensor 2 stuck at about 450 mV (or 1.9 V)
Common causes
- Signal or ground wire open, connector off or corroded Most likely
- Sensor heater failed, sensor never becomes active Possible
- Sensor worn out (no longer produces voltage) Possible
- Control module input faulty Less likely
How to diagnose P0140
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: oxygen sensor bank1 sensor2, oxygen sensor wiring, catalytic converter.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 1
- Fuel and air metering
- 40
- Specific fault
Questions drivers ask
Can I drive with code P0140?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0140?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0140 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0140 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.