Will a car run with a faulty camshaft position sensor?

Usually yes, but badly. The camshaft sensor tells the engine which stroke each cylinder is on, and without it most modern control units fall back on trying an injection pattern, seeing what happens and correcting. That works well enough to run the engine but it takes far longer to start, gives rough running and reduced power, and can stall. Some engines will not start at all without the signal.

What are the symptoms of a bad camshaft position sensor?

The classic signs are long cranking before the engine fires, stalling at idle or as you come to a stop, hesitation and lost power, and an engine management lamp with a reduced power warning on some cars. It is often intermittent at first, working when cold and failing when hot, or the reverse, which is why the fault can come and go for weeks before it settles into something constant.

Is P0340 the same as a timing chain problem?

Not usually, but it can point at one. P0340 means the sensor's signal is missing or unusable, which is most often the sensor, its connector or its wiring, and oil that has wicked along the wires into the plug is a very common finding. A timing chain that has jumped far enough can also put the signal somewhere the control unit does not expect, but that normally sets a camshaft-to-crankshaft correlation code such as P0016 alongside. If P0340 is the only code stored, look at the electrics first.

Will P0340 fail an MOT?

Yes. An illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, and the car fails on the lamp alone. There is an added complication with this code: the emissions test needs the engine running at idle and at a fast idle, so if the sensor fault stops the car starting or makes it stall, the check cannot be completed and that is a major failure of its own.

Powertrain code · standard

P0340No usable signal from the camshaft sensor

Standard definition: Camshaft Position Sensor "A" Circuit (Bank 1 or Single Sensor)

Last updated 27 July 2026

Quick answer

The control unit is not getting a signal it can use from the camshaft position sensor on bank one, or from the only camshaft sensor if the engine has just one. The usual result is long cranking and rough running rather than a car that will not move at all.

How the car works this out

The camshaft sensor tells the control unit which stroke each cylinder is on, something the crankshaft sensor cannot do by itself: the crankshaft turns twice for every single turn of the camshaft, so without the camshaft signal the unit cannot tell a compression stroke from an exhaust stroke. It watches for pulses arriving in the expected pattern as the engine turns, and stores the circuit code when there are no pulses at all, when they stop part way through, or when nothing appears where the crankshaft signal says it should. Most modern engines can still run without it by guessing the stroke, firing, and correcting from what happens, which is why the everyday symptom is a long crank rather than a dead car.

What you might notice

Cranking for noticeably longer than usual before the engine catches
Stalling at idle or when coming to a stop, sometimes restarting immediately afterwards
A car that will not start at all on some engines
Rough running, hesitation and reduced power, because the injection reverts to a cruder mode
The engine management lamp on, and on many cars a reduced power warning with it

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 itself, which is a wearing item on plenty of engines and the most likely single answerCommon
How to check: Watch camshaft speed on live data while the engine cranks. A reading stuck at zero, or jumping erratically, confirms it.
2The connector, particularly on engines where oil creeps along the wires from the sensor and fills the plugCommon
How to check: Unplug it and look inside the connector body for oil. Oil-wicked connectors are common and the sensor gets blamed for a great many of them.
3Chafed, broken or heat-damaged wiring between the sensor and the control unitPossible
4A damaged trigger wheel on the camshaft, or a sensor air gap altered by a loose mounting bolt or a missing spacerPossible
5Metallic debris or thick sludge stuck to the sensor tip, blinding itPossible
How to check: Remove the sensor and look at the tip. A magnetic sensor collects swarf, and a wipe can restore a signal long enough to prove the point.
6A timing chain that has jumped far enough that the signal no longer arrives anywhere near where it is expectedLess common
How to check: Look for a correlation code such as P0016 stored alongside. On its own, P0340 is far more likely to be electrical than mechanical.
7A failed supply or reference earth from the control unitLess common

What to do about it

1Unplug the sensor and inspect the connector before ordering a part. Oil inside the plug is a common finding and cleaning it, plus fixing whatever let the oil in, is a much smaller job than it looks.
2Have the live data checked while cranking. A camshaft speed reading of zero while the crankshaft reads normally is about as clear a result as diagnostics gets.
3Check whether a correlation code is stored alongside. P0340 on its own is usually a sensor or its wiring; P0340 with P0016 shifts the whole investigation towards the timing chain.
4Do not leave it because the car still starts eventually. A sensor that has become intermittent will stall the engine sooner or later, and it tends to choose the worst moment.

Can you keep driving?

Only to a garage

Most cars keep running with a failed camshaft sensor, but they do it by guessing, and an intermittent sensor can cut the engine without warning. Stalling at a junction or on a roundabout is the real risk here rather than any damage to the engine. Get it to a garage rather than commuting on it, and expect that it may not restart once you switch it off.

Will it fail an MOT?

Beyond the lamp, which is a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008, this code carries a specific practical problem: the car has to run for the test to happen. Emissions readings are taken with the engine at idle and again at a fast idle, so an engine that will not start, or that stalls on the ramp, cannot be measured. Depending on how far it gets, the tester will either be unable to complete the emissions check, which is a major failure, or unable to carry out the test at all.

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

Will a car run with a faulty camshaft position sensor?: Usually yes, but badly. The camshaft sensor tells the engine which stroke each cylinder is on, and without it most modern control units fall back on trying an injection pattern, seeing what happens and correcting. That works well enough to run the engine but it takes far longer to start, gives rough running and reduced power, and can stall. Some engines will not start at all without the signal.
What are the symptoms of a bad camshaft position sensor?: The classic signs are long cranking before the engine fires, stalling at idle or as you come to a stop, hesitation and lost power, and an engine management lamp with a reduced power warning on some cars. It is often intermittent at first, working when cold and failing when hot, or the reverse, which is why the fault can come and go for weeks before it settles into something constant.
Is P0340 the same as a timing chain problem?: Not usually, but it can point at one. P0340 means the sensor's signal is missing or unusable, which is most often the sensor, its connector or its wiring, and oil that has wicked along the wires into the plug is a very common finding. A timing chain that has jumped far enough can also put the signal somewhere the control unit does not expect, but that normally sets a camshaft-to-crankshaft correlation code such as P0016 alongside. If P0340 is the only code stored, look at the electrics first.
Will P0340 fail an MOT?: Yes. An illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, and the car fails on the lamp alone. There is an added complication with this code: the emissions test needs the engine running at idle and at a fast idle, so if the sensor fault stops the car starting or makes it stall, the check cannot be completed and that is a major failure of its own.

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