P0140: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
Key takeaways
- P0140 means the downstream oxygen sensor on bank 1, the one after the catalytic converter, is sending a flat signal that never moves.
- It is safe to drive; on most cars that sensor only checks the converter and does not set the fuel mixture.
- A dead sensor heater, a broken wire or a loose connector causes it more often than anything else; a new sensor runs $150–$400 installed.
- Chrysler traced some 3.6L V6 cases to oil leaking from the valve cover onto the sensor, so look for oil before replacing it.
What P0140 means
The oxygen sensor behind your catalytic converter has gone quiet: its signal sits flat instead of moving with the exhaust. Scanners call it O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2). Once the engine is warm, a working downstream sensor produces a voltage that moves between lean and rich readings. When the engine computer sees the signal parked near its own bias voltage (roughly half a volt on most cars) for too long, it decides the sensor is dead, disconnected or never warmed up, and sets P0140. Bank 1 is the side of the engine with cylinder 1; a four-cylinder has only bank 1. Sensor 2 is the one after the converter.
Is it safe to drive?
Yes. On most cars the downstream sensor only watches the converter; it doesn’t decide how much fuel the engine gets, so the car drives normally. Book the repair in the next few weeks: a lit light fails an OBD emissions inspection, the converter test can’t run, and the light hides any new code that sets. What changes the answer is a rough-running engine, a fuel smell, or a flashing check engine light. Those point to a different problem sharing the light, and it comes first.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Sensor heater dead, so the sensor never gets hot | P0141 set too; heater resistance open at the sensor | $150–$400 (new sensor; the heater is built in) |
| Blown O2 heater fuse | No power at the sensor connector with the key on | $0–$20 |
| Broken, melted or corroded wiring or connector | Wire touching the exhaust, green pins; signal returns when wiggled | $100–$250 |
| Worn-out sensor | Circuit and heater test good, but the signal stays flat on a scan tool | $150–$400 |
| Oil or coolant fouling the sensor | Oily or white-crusted sensor tip; leak above the exhaust | $150–$500 for the leak (valve cover gasket) plus a sensor if fouled |
| Exhaust leak ahead of the sensor | Ticking at cold start, soot at a joint | $150–$600 |
The first three are the usual answers, and the fuse and wiring are the cheapest, so check those before buying a sensor.
What to check first
- Read every code. P0140 with P0141 points to the heater circuit. P0140 with P0137 or P0138 points to the signal wire or the sensor itself.
- Check the O2 heater fuse. If it’s blown, replace it once; if it blows again, find the short.
- Follow the harness from the sensor to its plug. Look for wires resting on the pipe, broken clips and corrosion in the connector.
- Watch the sensor on a scan tool at warm idle and after a snap of the throttle. A dead-flat line while the upstream sensor switches normally means the sensor or its circuit.
- Look for oil above the sensor before fitting a new one.
- Clear it and drive. After the repair, drive a few days of mixed trips so the sensor and converter tests run.
The bulletins show a pattern. Subaru released new downstream oxygen sensors for 2020–2023 Outbacks with P0140 and related codes. Chrysler traced some 2015–2017 Chrysler 200 3.6L V6 codes to oil leaking from the valve cover onto the sensor. GM told technicians to check for a bent terminal on some 2013 Silverado and Sierra 2500 trucks, and has a separate note for 2013–2015 Acadia, Enclave and Traverse.
What it costs
The cheap outcome is a fuse, under $20. The common outcome is a new downstream sensor at $150–$400 installed, or a wiring repair at $100–$250. The expensive outcome is fixing an oil leak above the sensor, $150–$500 for a valve cover gasket, plus the sensor if the old one was fouled. A diagnostic hour ($100–$180) tells a dead sensor from a broken wire before you buy parts. Our O2 sensor check engine light guide covers the rest of the sensor family.
Related codes
P0140 often comes with P0141 (the same sensor’s heater), and sometimes with P0137 or P0138. On a V6 or V8 the bank 2 version is P0146. If P0420 appears after the repair, give the converter test a few drive cycles before blaming the converter, because it depends on this sensor. 7 complaints filed with NHTSA mention P0140, across 6 models; those are owner reports, not confirmed causes. If the car otherwise runs fine, see check engine light on but car runs fine.
What owners report to NHTSA
7 complaints filed with NHTSA mention P0140, 1 of them since 2015, across 6 different models. The component owners most often filed it under: electrical system.
Manufacturer service bulletins that name P0140
13 technical service bulletins and manufacturer communications filed with NHTSA mention P0140, from 7 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- TRIUMPH SCRAMBLER 1200 X (2024), bulletin 10251741: Dealer notice advising that models fitted with catalyst monitoring, may trigger DTC P0140 on 2 situations.
- SUBARU OUTBACK (2020, 2021, 2022, 2023), bulletin 11000828: This bulletin announces the availability of new oxygen sensors developed to address the following Diagnostic Trouble Codes (DTCs); P0137 (O2 Sensor Circuit Low Voltage Bank 1 Sensor 2, P0138 (O2 Sensor Circuit High Volta…
- CHRYSLER 200 (2015, 2016, 2017), bulletin 10224882: Valve Cover and Gasket, Left and Right Pertains to vehicles equipped with a 3.6L V6 V.V.T. Engine only: Oil leaking from the valve cover onto the O2 sensor causing faults codes P0140, P2096, P0137, P0157. Need to verify …
- LAND ROVER RANGE ROVER (2016), bulletin 10105881: "ISSUE: THE CUSTOMER MAY OBSERVE A MIL IN THE INSTRUMENT CLUSTER RELATING TO THE ENGINE SYSTEM (CHECK ENGINE LIGHT) AND ONE OR MORE CODES STORED IN THE PCM RELATING TO HEATED OXYGEN SENSOR (HO2S) IN THE EXHAUST SYSTEM. T…
- GMC ACADIA (2013, 2014, 2015), bulletin 10135332: This Preliminary Information communication provides information to the technician about vehicles that have a Service Engine Soon Malfunction Indicator Lamp on. Technician may find a Diagnostic Trouble Codes P0030, P0031,…
- BUICK ENCLAVE (2013, 2014, 2015), bulletin 10135332: This Preliminary Information communication provides information to the technician about vehicles that have a Service Engine Soon Malfunction Indicator Lamp on. Technician may find a Diagnostic Trouble Codes P0030, P0031,…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can I drive with P0140?
Yes. The sensor after the converter doesn't run the engine on most cars, so the car drives the same. Fix it before your next emissions test, because the light fails an OBD inspection and the converter monitor can't finish without this sensor.
Is P0140 the same as P0141?
No, but they are close. P0141 is a fault in the sensor's heater circuit; P0140 means the signal itself is flat. A dead heater keeps the sensor cold, and a cold sensor sends no signal, so the two often set together.
Will a new O2 sensor fix P0140?
Often, but check the fuse, the connector and the wire along the exhaust first. If oil or coolant fouled the old sensor, a new one will fail the same way unless you fix the leak.
Where is bank 1 sensor 2?
Bank 1 is the side of the engine with cylinder 1; a four-cylinder has only bank 1. Sensor 2 is the one after the catalytic converter, under the car.