What is the camshaft position sensor, how it works and bad symptoms

In modern internal combustion engines, the electronic control unit (ECU) controls the opening of the injectors, the spark production, the timing phases (in variable-timing engines), depending on the position of the pistons in the cylinders. The ECU must recognize the position of each piston in the cylinder according to the crankshaft speed signal and/or camshaft position signal.

The mechanical position of the pistons in the cylinders, during engine operation, can be determined based on the position of the crankshaft and camshaft. With this information, the electronic control unit performs engine synchronization (piston position recognition).

On a 4-cylinder in-line engine, for example, the usual ignition order is 1-3-4-2. From a geometrical point of view, in these engines, 2 pistons are offset by 180 ° compared to the other 2.

How does the camshaft position sensor work?

To summarize how the camshaft position sensor works, the short answer is that the sensor allows the ECU computer to determine the exact position of the camshaft.

This information is extremely important and helps in calculating the ignition point of the fuel but also its injection into the cylinder. 

The information passed by the sensor is necessary for the beginning of the fuel injection sequence but also for the selective control of the cylinders. 

The camshaft sensor is a very important part that determines the operation or malfunction of the car. He tells the spark plugs to turn on the fuel. If it does not work properly the car will fail or in some cases, it will not start at all. 

A complete explanation of camshaft position sensor functioning is that during engine operation, when pistons 1 and 4 will be at BDC, pistons 2 and 3 will be at TDC

What are BDC and TDC?

To summarize:


  • is the position in which the piston is closest to the cylinder head, and the axis of the connecting rod is in continuation of the piston axis
  • is the point where the piston speed is zero
  • is the position of the piston at which corresponds to the minimum volume occupied by the motor fluid in the cylinder


  • is the position where the piston is the furthest from the cylinder head and the axis of the connecting rod is the continuation of the piston axis
  • is the point where the piston speed is zero
  • is the position of the piston at which corresponds to the maximum volume occupied by the motor fluid in the cylinder

In this case, when pistons 1 and 4 are at the BDC, one piston is at the end of the compression stroke and the second at the end of the exhaust stroke. Ignition/injection should only be done at the piston at the end of the compression stroke.

In older engines, with mechanical ignition (petrol) or mechanical injection pump (diesel), the synchronization of the ignition/injection system with the position of the pistons in the cylinders is performed automatically, due to the direct mechanical connection with the crankshaft.

In modern engines, where ignition and injection are controlled by the ECU, synchronization is not automatic but must be done based on information from the sensors. The piston position synchronization can be done in two ways:

  • using only crankshaft position information (engine speed)
  • using both crankshaft position and camshaft position information

The first method has the advantage that it does not require an additional position sensor on the camshaft. The ECU contains algorithms that synchronize the position of the pistons through successive tests. In other words, if 2 pistons are found in the BDC, the injection computer will order the fuel injection sequentially for each cylinder.

Then with the help of the crankshaft position sensor will deduce which piston produces torque (measured by increasing the speed), where it turns out that the first piston was at BDC and the second at TDC.

The disadvantage of this method is the greater engine start-up time. Also, if the crankshaft position sensor (engine speed) fails, starting the engine is impossible. This method does not apply to motors with variable timing because camshaft position information is required for these.

The second method, more commonly used, requires the use of a camshaft position sensor. Most sensors work on the Hall effect principle. The camshaft position is read using a metal wheel (pin) fixed to it.

Depending on the type of engine, there are several variants for reading the camshaft position. Metal pins or metal wheels with variable numbers and lengths of teeth can be used.

By using the signals of the two sensors (crankshaft position and camshaft position), the injection computer can determine which piston is on the intake stroke and which is on the exhaust stroke (in the case of a 4-cylinder in-line engine.

The crankshaft position sensor detects when the pistons are at BDC (eg 1 and 4). Then, with the position signal from the camshaft (positive or null), determine which of the two pistons is on the compression stroke.

In the case of using a metal wheel with multiple teeth, with variable length, the synchronization is faster, and in case of failure of the motor position sensor, the degraded operation of the motor is ensured by the camshaft position sensor

Where is the camshaft position sensor located?

Depending on the type of engine, the camshaft position sensor is usually mounted on the cylinder head cover. In variable-timing engines, the gearwheels are positioned on the camshaft position change system. You will find it 100% if you identify the position of the camshaft.

The camshaft position sensor that works on the Hall effect principle has 3 pins:

  • power supply (+5 V)
  • ground (0 V)
  • position signal (+13.5 V)

The OBD 2 fault codes for the camshaft position sensor are P03xx.

Symptoms of a bad camshaft position sensor

  • starting problems;
  • check engine error icon appears on board;
  • interruptions in connection with the on-board computer;
  • In happy cases, the engine does not pass over 2000 RPM;
  • engine misfires

Is it okay to drive if you have a bad camshaft sensor?

If the car has the above symptoms, do NOT drive the car anymore, you risk ruining more parts, even quite expensive parts such as the catalytic converter.

Camshaft sensor cleaning

In most cases, the camshaft sensor is only dirty when the car starts to show the above symptoms.

Below are some steps you can take to clean the camshaft sensor.

  • Disconnects the minus wire from the car battery.
  • After you do this you need to locate the sensor, as I told you above. It is not in the same place on all cars, so it would be best to use the manual of the car and you will find it much faster.
  • The sensor has 3 cables, note on paper each one where it comes in the initial position to mount them after the correct place.
  • Use a screwdriver and remove the sensor, put it on a clean cloth.
  • Clean it now with a contact spray, then wipe it with another clean cloth.
  • Put it back, connect the 3 cables in the initial position, reconnect the car battery and it’s done.

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