What does no activity detected mean on an oxygen sensor?

It means the signal has not moved. The control unit puts a small bias voltage of around half a volt onto the sensor's signal wire, and a working sensor overrides that with its own constantly changing output. If the reading simply sits at the bias value and never varies, the circuit is broken, the sensor is dead, or it is too cold to generate anything. That is a distinct observation from a signal that is stuck high or stuck low, which is why it has its own code.

Will P0140 fail an MOT?

Yes, and on two fronts. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003. Separately, with no usable signal from the front sensor the engine has no closed-loop mixture control and runs from a fixed, deliberately rich map, so the calculated lambda at fast idle has a real chance of falling outside the required 0.97 to 1.03 band. This is one of the codes that can fail the gas test on its own, which also puts older cars outside the lamp rule at risk.

Can a bad oxygen sensor heater cause P0140?

Yes, and the two codes frequently appear together. A zirconia sensor generates no signal at all until its element is several hundred degrees Celsius, so if the heater has failed the sensor may never reach that temperature during ordinary driving, particularly on short journeys. The control unit sees a flat signal and records no activity, when in fact the measuring element is perfectly serviceable and simply cold. Always check whether a heater circuit code is stored before deciding what has failed.

Why is my fuel economy worse with a P0140?

Because the engine has lost its feedback. Normally the front sensor tells the control unit whether the last mixture was rich or lean so it can correct continuously. Without a signal the unit falls back on a fixed lookup table, and that fallback is deliberately calibrated slightly rich, since running a little too much fuel is safer for the engine than running too little. The result is more fuel used and a harder life for the catalytic converter until the sensor is put right.

Powertrain code · standard

P0140Front oxygen sensor showing no signal at all

Standard definition: O2 Sensor Circuit No Activity / Slow Response (Bank 1 Sensor 2)

Last updated 27 July 2026

Quick answer

The signal from the sensor before the catalytic converter is not moving. Not high, not low, not slow — simply flat. That pattern points at an electrical break or a sensor that is not being brought up to temperature, and a sensor left unplugged after exhaust work is a genuinely common explanation.

How the car works this out

The control unit applies a small bias voltage of around half a volt to the sensor's signal wire. A live sensor overrides that bias with its own output, which swings up and down continuously. If the circuit is broken, or the sensor is dead, or it never reaches operating temperature, nothing overrides the bias and the reading simply sits at that fixed value indefinitely. The control unit sees a signal that has not moved at all over a period when it certainly should have, and records no activity. That is a different observation from a signal stuck at the top or bottom of its range, which is why this code exists separately.

What you might notice

Fuel consumption clearly worse, because the control unit is running from a fixed table rather than sensor feedback
A slightly rich exhaust smell
Occasionally a hesitation or a lack of smoothness under light load
Frequently no driveability symptoms at all beyond the lamp

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The sensor left unplugged, or its connector not properly latched, after exhaust, clutch or gearbox workCommon
How to check: Look at the connector. It costs nothing, and if the car has been in a workshop recently it is the first thing to check.
2A broken or chafed signal wire, or a connector corroded to the point of an open circuitCommon
How to check: Unplug at the sensor and check the harness side for the bias voltage of around half a volt. Present means the wiring and control unit are good; absent means the fault is further back.
3An internally open-circuit sensor at the end of its lifeCommon
4A failed sensor heater, so the element never gets hot enough to generate any signal at allCommon
How to check: Check whether a heater circuit code is stored alongside this one. The pairing is common and it changes the diagnosis, because the signal side may be perfectly healthy.
5A large exhaust leak ahead of the sensor letting so much cold air past that the element cannot stay at temperaturePossible
6A poor earth or a supply fault shared with other sensorsLess common

What to do about it

1Check the connector physically first, especially if the car has recently had work done anywhere near the exhaust. An unplugged sensor is embarrassing to find and cheap to fix.
2Look for a heater code stored at the same time. A sensor that is stone cold produces no signal, and replacing the signal side would be the wrong repair.
3Check for the bias voltage at the harness with the sensor unplugged. That single reading tells you which side of the connector the problem is on.
4Do not leave this one running for months. With no usable signal the control unit fuels from a default table, which is deliberately set on the rich side, and that is hard on the catalytic converter and on fuel bills.
5Have the exhaust checked for leaks if the wiring and sensor both test sound.

Can you keep driving?

Yes

The car will drive, and on many vehicles it drives almost normally, because the control unit falls back on a pre-programmed fuelling map. That fallback is deliberately biased slightly rich for safety, so you will use more fuel than usual and the catalytic converter will be working harder than it should. It is safe for the time being, but it is not a state to leave the car in for months.

Will it fail an MOT?

This is one of the emissions codes with the strongest chance of failing the gas test on its own merits, quite separately from the lamp. Without a usable signal from the front sensor there is no closed-loop mixture control at all — the engine is fuelling from a fixed map, which is set deliberately rich as a safety margin. The tester's calculated lambda at fast idle has to fall between 0.97 and 1.03, and an engine running open loop has nothing keeping it there. So where many codes here fail purely on the lamp, this one can produce a genuinely failed emissions reading too. The lamp remains a major defect for petrol cars first used on or after 1 July 2003, and older cars outside the lamp rule are still exposed through the gas readings.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What does no activity detected mean on an oxygen sensor?: It means the signal has not moved. The control unit puts a small bias voltage of around half a volt onto the sensor's signal wire, and a working sensor overrides that with its own constantly changing output. If the reading simply sits at the bias value and never varies, the circuit is broken, the sensor is dead, or it is too cold to generate anything. That is a distinct observation from a signal that is stuck high or stuck low, which is why it has its own code.
Will P0140 fail an MOT?: Yes, and on two fronts. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003. Separately, with no usable signal from the front sensor the engine has no closed-loop mixture control and runs from a fixed, deliberately rich map, so the calculated lambda at fast idle has a real chance of falling outside the required 0.97 to 1.03 band. This is one of the codes that can fail the gas test on its own, which also puts older cars outside the lamp rule at risk.
Can a bad oxygen sensor heater cause P0140?: Yes, and the two codes frequently appear together. A zirconia sensor generates no signal at all until its element is several hundred degrees Celsius, so if the heater has failed the sensor may never reach that temperature during ordinary driving, particularly on short journeys. The control unit sees a flat signal and records no activity, when in fact the measuring element is perfectly serviceable and simply cold. Always check whether a heater circuit code is stored before deciding what has failed.
Why is my fuel economy worse with a P0140?: Because the engine has lost its feedback. Normally the front sensor tells the control unit whether the last mixture was rich or lean so it can correct continuously. Without a signal the unit falls back on a fixed lookup table, and that fallback is deliberately calibrated slightly rich, since running a little too much fuel is safer for the engine than running too little. The result is more fuel used and a harder life for the catalytic converter until the sensor is put right.

Check any vehicle's MOT history

Free, instant, and straight from DVSA records. No sign-up.