What does circuit low input mean on a fault code?

It means the control unit is seeing a voltage on that circuit that is at or near earth when it should be higher. It is an electrical measurement, not a judgement about how the engine is running, and it points at three things: a wire shorted to earth somewhere along its length, a sensor that has failed with its output pulled low, or a supply voltage that is not reaching the sensor so the output never rises. On a camshaft sensor circuit, a chafed wire or a connector full of oil accounts for a large share of them.

Is P0342 the sensor or the wiring?

It can be either, and the test that separates them takes minutes. Unplug the sensor and measure the signal wire to earth with the sensor out of circuit: if it still reads as a short, the fault is in the harness and a new sensor would achieve nothing. If it reads correctly with the sensor unplugged, the sensor is pulling its own output low and needs replacing. Also look inside the connector, because oil wicking along the wires from a leaking seal bridges the pins and produces exactly this code.

Can P0342 stop my car starting?

It can. The camshaft signal tells the engine which stroke each cylinder is on, and while most modern engines can start without it by guessing and correcting, some will not, and all of them take noticeably longer. Stalling over bumps and refusing to restart once switched off are common with this code, because the underlying fault is often an intermittent short that comes and goes with vibration.

Will P0342 fail an MOT?

Yes. Once the fault has been confirmed the engine management lamp stays lit, and an illuminated lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. It makes no difference that the fault is intermittent or that the car drives normally on the day of the test.

Powertrain code · standard

P0342Camshaft sensor signal stuck low

Standard definition: Camshaft Position Sensor "A" Circuit Low Input (Bank 1)

Last updated 27 July 2026

Quick answer

The voltage on the camshaft sensor's signal wire is sitting lower than it should — either pulled down to earth or never rising in the first place. It is the most specifically electrical of the camshaft sensor codes, which also makes it one of the more findable: the fault is usually in a connector or a length of wire rather than inside the engine.

How the car works this out

The control unit supplies the sensor and watches the voltage on the signal wire, which should switch cleanly between a high and a low level as the trigger wheel passes. A low-input code is set when that voltage sits at or near earth and never rises, either continuously or for longer than a permitted period. It is a straightforward electrical judgement with no reference to timing, which is why P0342 says nothing about the state of your chain. What it does say is that current is going somewhere it should not, or that the voltage that should be there is not arriving.

What you might notice

Long cranking before the engine starts, or a no-start
Stalling, particularly over bumps or when the harness is disturbed
Rough running and lost power, with the injection running in a fallback mode
A fault that comes and goes with weather, temperature or road surface
The engine management lamp on, sometimes appearing and clearing repeatedly

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.

1A short to earth in the signal wire, typically where the harness rubs against a bracket, a heat shield or the edge of the cam coverCommon
How to check: Unplug the sensor and measure the signal wire to earth with the sensor out of circuit. A dead short with the sensor disconnected proves the fault is in the wiring, not the part.
2Water or oil inside the sensor connector bridging the pinsCommon
How to check: Unplug and inspect. Oil that has wicked along the wires from a leaking sensor seal is common, and drying the connector often restores the signal temporarily, which confirms the diagnosis.
3A failed sensor pulling its own output lowCommon
4A missing supply or reference voltage to the sensor, so its output never risesPossible
How to check: Back-probe the supply pin with the ignition on and compare against the specified voltage, which is usually five or twelve volts depending on the sensor type.
5Corroded, spread or backed-out terminals in the connectorPossible
6A damaged input inside the control unitLess common

What to do about it

1Start at the connector. Unplug it, look for oil, water and green corrosion, and check that every pin is seated at the same depth.
2Have the wiring tested rather than the sensor replaced first. A short to earth will destroy a new sensor just as happily as it silenced the old one.
3Trace the harness where it crosses the cylinder head and passes any bracket or heat shield. That is where the insulation gives way, and the damage is usually visible once you look.
4Fix whatever let oil into the connector at the same time, or the repair will not last. A leaking sensor O-ring or cam cover gasket is what starts most of these.

Can you keep driving?

Only to a garage

The car will often run, but a wiring fault of this kind is unpredictable in a way a worn component is not. An intermittent short can cut the engine over a bump, which is the practical hazard rather than any damage to the engine itself. Get it to a garage rather than living with it, and be aware it may not restart once you switch off.

Will it fail an MOT?

Intermittent faults still fail the test, and this is one of the most intermittent codes in the family. Once a fault has been confirmed the lamp stays lit whether or not the wiring is behaving on the morning of the test, and an illuminated malfunction indicator lamp is a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008. A car that drives perfectly on the day and has never once stalled in front of its owner will still be handed a failure certificate for the lamp alone.

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 circuit low input mean on a fault code?: It means the control unit is seeing a voltage on that circuit that is at or near earth when it should be higher. It is an electrical measurement, not a judgement about how the engine is running, and it points at three things: a wire shorted to earth somewhere along its length, a sensor that has failed with its output pulled low, or a supply voltage that is not reaching the sensor so the output never rises. On a camshaft sensor circuit, a chafed wire or a connector full of oil accounts for a large share of them.
Is P0342 the sensor or the wiring?: It can be either, and the test that separates them takes minutes. Unplug the sensor and measure the signal wire to earth with the sensor out of circuit: if it still reads as a short, the fault is in the harness and a new sensor would achieve nothing. If it reads correctly with the sensor unplugged, the sensor is pulling its own output low and needs replacing. Also look inside the connector, because oil wicking along the wires from a leaking seal bridges the pins and produces exactly this code.
Can P0342 stop my car starting?: It can. The camshaft signal tells the engine which stroke each cylinder is on, and while most modern engines can start without it by guessing and correcting, some will not, and all of them take noticeably longer. Stalling over bumps and refusing to restart once switched off are common with this code, because the underlying fault is often an intermittent short that comes and goes with vibration.
Will P0342 fail an MOT?: Yes. Once the fault has been confirmed the engine management lamp stays lit, and an illuminated lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. It makes no difference that the fault is intermittent or that the car drives normally on the day of the test.

Check any vehicle's MOT history

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