P0140 Code: O2 Sensor No Activity Detected (Bank 1 Sensor 2)
The oxygen sensor after the converter on bank 1 shows a dead-flat signal that never reacts.
At a glance
Severity: Low
- Meaning
- O2 Sensor No Activity Detected (Bank 1 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$280
- DIY difficulty
- Easy
A downstream sensor behind a good converter is supposed to be calm, so the PCM doesn't flag it simply for holding steady. What sets P0140 is a signal locked inside a narrow window around the PCM's own bias voltage, typically about 0.45 V, that doesn't move even when the PCM expects a reaction, such as during a deceleration fuel cut or a sustained rich load. That flat bias line means the sensor isn't contributing anything: it is too cold to work because its heater has failed, the element is dead, or the signal or ground wire is open so the PCM is effectively looking at an empty input. Because the rear sensor and its harness hang under the car, broken wires from road debris, careless lifting or exhaust work are common, and a connector left unplugged after a converter replacement is a classic cause.
Can I keep driving? The car drives normally and there's no mechanical risk. The catalyst monitor can't run without this sensor, so repair it before any emissions test.
How to read P0140
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 40 Specific fault
Symptoms
- Check engine light, with the car driving as usual
- B1S2 frozen at about 0.43 to 0.47 V whatever the driving condition
- No drop during coast-down and no rise under hard acceleration
- Catalyst monitor stays incomplete
- Often appears together with the rear heater code P0141
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Open signal or ground wire under the car: snagged by debris, pinched during lifting or torn during exhaust work
- Heater failed, so the sensor stays too cold in the cooler post-converter exhaust
- Sensor element dead
- Connector left unplugged or loose after converter or exhaust replacement
- Blown fuse for the sensor heater circuit
- Water in the connector corroding a pin through
- PCM input or bias circuit fault
Diagnosis step by step
Tell 'steady' apart from 'dead'
Graph B1S2 on a warm drive. A healthy rear sensor sits somewhere around 0.5 to 0.8 V, falls toward 0.1 V when you coast in gear and rises under a long hard acceleration. A line pinned at about 0.45 V through all of that is the PCM's bias with no sensor behind it.
Tools: OBD-II scanner
Check for heater codes first
If P0141, P0037 or P0038 is stored, the sensor is probably just cold; diagnose the heater circuit before anything else. No heater codes shift suspicion to the signal wiring and the element.
Tools: OBD-II scanner
Inspect connector and harness under the car
Raise the car safely and follow the pigtail from the sensor to its connector. Look for a plug that isn't latched, a harness stretched tight after exhaust work, wires torn off at a clip, and green corrosion eating through a pin.
Tools: Flashlight
Prove the wiring with a ground test
Unplug the sensor, key on, and watch B1S2 on the scan tool: it should show the bias. Briefly connect the harness-side signal pin to the harness-side sensor ground pin with a jumper; the reading must drop to near 0 V. If it doesn't move, the wire to the PCM or the ground is open.
Tools: Multimeter, OBD-II scanner
Bench-test the sensor with a flame
Remove the sensor and connect a meter to its signal and ground wires. Heat the tip with a propane torch: inside the flame, where oxygen is used up, a working sensor produces around 0.8 to 0.9 V, and it drops toward 0.1 V as soon as you pull the flame away. No voltage at all means the element is dead.
Tools: Multimeter, O2 sensor socket
Install and secure
Fit the new sensor at about 30 lb-ft, route the pigtail with enough slack for engine and exhaust movement, and fix it in every original clip away from the converter shell. Confirm on a drive that the rear sensor now reacts to coast-downs.
Tools: Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Plug in and latch the rear sensor connector | Yes | $0–$20 |
| Repair torn or open sensor wiring | No | $80–$250 |
| Replace downstream O2 sensor yourself (part) | Yes | $40–$130 |
| Replace downstream O2 sensor at a shop | No | $120–$280 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
With P0140 stored, the check engine light is on and the catalyst monitor can't complete, so an OBD-II emissions test will fail twice over. Fix the circuit first, then drive normally for several days before the retest.
Frequently asked questions
Isn't the rear O2 sensor supposed to be steady?
Steady, yes, but not dead. A good rear sensor sits in the upper half of its range and still reacts to coasting and hard acceleration. A line frozen right at the bias voltage means the PCM isn't hearing the sensor at all.
I just had a new catalytic converter fitted and now have P0140. Why?
The rear sensor has to be unplugged or moved to the new converter, and it is easy to leave the connector loose or stretch the pigtail. Check the plug and harness before anything else.
Can I test the sensor without special tools?
A simple bench test works: connect a multimeter to the signal and ground wires and heat the tip with a propane torch. A healthy sensor shows close to 0.9 V in the flame and drops quickly when you take the flame away.
What is the difference between P0140 and P0134?
Both mean no activity, but P0134 is the front sensor that controls fueling, so it affects fuel economy. P0140 is the rear sensor, so it mostly affects the catalyst monitor and the emissions test.