P0140
PowertrainO2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
Most people who get this never notice anything other than the engine warning light popping up. The car drives fine, pulls fine, and nothing feels off. What's happened is the ECU has stopped seeing any signal at all from the rear oxygen sensor on bank 1, the one sitting after the catalytic converter. That sensor's job is to keep an eye on how well the cat is cleaning the exhaust, so when it goes completely dead the ECU flags it and lights the dash.
ⓘ Information only. This page provides general educational information about fault code P0140. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0140 mean?
P0140 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, very often the only thing you'll spot
- • No real change in how the car drives, which is why people ignore it for months
- • Slightly worse fuel economy in some cases, though it's rarely dramatic
- • Faint rough running or hesitation when cold on a few engines
- • On a failed heater circuit, the light may take a few minutes to appear after a cold start
Possible causes
Causes commonly associated with P0140, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Dead oxygen sensor itself, far and away the most common cause. The heater element inside burns out or the sensor element gets contaminated, and high-mileage cars over 90,000 miles are prime candidates
- 2. Corroded or damaged wiring at the sensor connector. These sensors live under the car in the road spray, so the plug and the first few inches of loom take a battering. Check for green crusty pins
- 3. Blown fuse feeding the heater circuit. A simple one to overlook, worth a two-minute check before you condemn anything
- 4. Exhaust leak ahead of the sensor letting fresh air in and skewing the reading
- 5. Bad earth or open in the heater circuit ground, which stops the sensor reaching operating temperature and producing a signal
- 6. Failed ECU sensor input circuit. Rare, and you only land here once everything else checks out clean
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read live data and watch the bank 1 sensor 2 voltage. A working rear sensor should hover and switch lazily somewhere near 0.45V once warm. If it's pinned flat with no movement at all, that lines up with P0140
- 2. Pull up the connector and look hard at the pins and the loom for green corrosion, melted insulation, or a plug that's worked loose. This catches a fair share of these without spending a penny on parts
- 3. Check the heater circuit. You want roughly 12V on the supply side with the key on, and a clean ground. No heater means the sensor never gets warm enough to work
- 4. Check the fuse that feeds the sensor heater. If a circuit is dead across the board, a blown fuse is the cheapest possible answer
- 5. Look and listen for an exhaust leak between the engine and the sensor. A small leak forward of the sensor pulls in air and confuses the reading
- 6. If wiring, fuse, and heater all test fine, the sensor itself is the call. Replace it and clear the code
Common questions about P0140
How do I know if it's the sensor or just the wiring on my car? +
Start with the connector before you buy a sensor. Unplug it and check the pins for corrosion and the loom for chafing, because these sit under the car and get hammered by road salt. Then test the heater circuit: key on, you should see around 12V on the supply and a good ground. If the wiring and heater feed are healthy but the sensor voltage on your scan tool stays dead flat, the sensor itself has died. If the supply or ground is missing, you've got a wiring or fuse problem and a new sensor won't fix a thing.
Can I just replace it myself, or is that a garage job? +
Plenty of people do this on their own driveway. The part is usually £30 to £90 for a decent brand, and fitting is just unplugging the old one and unscrewing it. The catch is access and seizure. The rear sensor lives under the car, so you need it safely on stands or a ramp, and if it's been baked into the exhaust for years it can fight you. A proper oxygen sensor socket and a bit of penetrating fluid the night before helps. Cross-threading the new one into the bung is the main way people make this worse, so go in by hand first.
If I clear the code will it stay gone? +
If the sensor or wiring is actually faulty, clearing it just buys you a few miles before it comes back, usually within a drive cycle or two once the ECU rechecks the heater circuit. A genuine dead sensor won't heal itself. The only time clearing it sticks is if the fault was a loose connector or a one-off glitch and you've sorted the connection. Fix the cause, then clear it, then drive it for a bit to confirm it stays off.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →