Engine Troubleshoot

What Are The Symptoms Of A Bad Camshaft Position Sensor?

symptoms of a bad camshaft position sensor
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Written by The Motor Guy

Every petrol or diesel car has a camshaft position sensor

It plays a vital role in keeping your car or truck engine running smoothly by relaying information back to the ECU.

The symptoms of a bad camshaft position sensor can be similar to other common engine problems.

So it's important to learn how a camshaft position sensor works, and how to properly diagnose a suspected bad or failing one when necessary.

What is a Camshaft Position Sensor?

The Camshaft Position Sensor (also known as the CMP sensor) is one of the key sensors when it comes to the smooth running of your engine. 

The information it generates plays a crucial role in determining when each injector should be activated on a modern sequential fuel injection systems (SFI). The camshaft position sensor is responsible for sending the ECU a signal each time the cylinder #1 intake valve is opening. That information is then used in conjunction with the Crankshaft Position Sensor (CKP) signal for knowing when the rest of the cylinders’ intake valves are opening.

How Does A Camshaft Position Sensor Work?

Camshaft Position Sensors use the same electronic principles of other engine sensors such as Crankshaft Position Sensors.

In other words, they are so similar in construction they are often difficult to differentiate from one another when outside the vehicle.

There are two common types of sensors. The design can be based on “optical”, “inductive”, or “Hall Effect” principles. For the purpose of this article, we will explain the Hall Effect design.

Hall Effect sensors consist of two key elements, the reluctor ring (toothed wheel) and the sensor itself which internally has a permanent magnet.

camshaft position sensor

The reluctor is attached to the camshaft in a way that cylinder #1 intake valve opening coincides with the “tooth”.

Many designs include all cylinders (not only cylinder #1) but using a wider tooth for differentiating #1 which will always be the “reference”.

As the camshaft rotates the permanent magnet inside the sensor will induce an electrical current each time the tooth aligns with it. The Hall sensor signal produced this way is similar to a square waveform like the green like shown below: 

symptoms of bad camshaft sensor

The ECU uses this information for the fuel injectors “firing” sequence, thus the importance of an accurate reading.

As said earlier, independently of the Camshaft Position Sensor design (single tooth or multi-tooth) the Camshaft Position Sensor and Crankshaft Position Sensor signals are always compared and used together not only for fuel delivery but also for ignition timing and spark plug firing.


7 Symptoms of a Failing Camshaft Position Sensor

The most common symptoms of a bad Camshaft Position Sensor are:

1. Check Engine Light (CEL) lit

Due to its crucial role in the fuel delivery expect the CEL lit after a few driving cycles with a damaged CMP sensor.

2. Poor fuel economy

Without proper information coming from the CMP the ECU won’t operate efficiently and thus, a noticeable decrease in fuel economy will be evident.

3. Rough idle

Depending on the vehicle year and maker the ECU may not compensate for this failure during idle. This might cause a rough idle or even an engine dying occasionally while idling.

4. Engine hesitation during acceleration:

A bad Camshaft Position Sensor will make hard for the ECU determining the fuel injector firing sequence thus expect a huge drop in performance. 

5. Engine Stall 

Depending on the emergency mode programmed on the ECU, you may experience intermittent engine stall.

6. Increased emissions 

A bad Camshaft Position Sensor will increase all emissions which could be a problem if you live in an area with smog checks.

7. Gas smell from exhaust pipe 

The failure to deliver the exact amount of fuel to the engine will cause a noticeable gas smell coming from your exhaust pipe.


How do you diagnose a failing Camshaft Position Sensor

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

Depending on the type of Camshaft Position Sensor installed on your vehicle, you may need one or more of the following tools:

  • OBD2 Code Reader
  • Digital multimeter (DMM)
  • Automotive diagnostic scanner with live data capabilities

A professional grade automotive scanner with live data graphing capabilities is not essential for testing the CMP sensor but certainly would speed up the process no matter what type of sensor is installed. 

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

Recommended tools: OBD2 Code Reader / automotive scan tool

> Data trouble codes: using an OBD2 code reader take note of any DTC present in the ECU memory. For doing so you only need to connect your code reader and turn the ignition key on (engine off), a state usually known as KOEO.

> Data trouble codes clear: now that you know what codes were stored into memory you need to clear them. This is usually done selecting “clear codes” in your tool.

> Driving cycle: disconnect your OBD2 code reader and start the engine, this state is commonly known as KOER (Key On Engine Running). If the Check Engine Light turns on then stop the engine and continue with the diagnostic. If the Check Engine Light remains off then drive the vehicle for 5 to 10 minutes. If the light remains off you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

IMPORTANT: The following tests assume the CMP sensor harness is already unplugged, refer to “how to replace” section if you have any doubt.

2. Visual Inspection

Recommended tools: none

> Sensor wiring: perform a meticulous visual inspection of CMP sensor wiring. Look for burnt, damaged, corroded or deteriorated wires. Pay special attention to possible shorts to ground or open circuits produced by faulty insulation.

> Sensor condition: since the CMP has a permanent magnet it can be clogged by metal particles that affect its readings. Clean the sensor using a soft soap and water. Reinstall and repeat the preliminary steps.

> Sensor harness connector: similar than before, perform an exhaustive visual inspection of CMP sensor connector.  Ensure the ignition key is switched off and unplug the CMP sensor harness connector. Look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

Fix any wiring problem before continuing.

how to diagnose bad camshaft position sensor

Overview of how all of the components of a modern fuel injected engine interact with one another

3. Electrical Tests

Recommended tools: digital multimeter (DMM), oscilloscope.

> Supply voltage: Hall Effect sensors use a three-wire design. Turn the ignition switch on (engine off) and look for the corresponding supply voltage (12V). Refer to the appropriate literature to identify the supply line.

> Sensor Signal: for this test, you will need using an oscilloscope. Place one tip on a known ground and the other test tip to the sensor signal wire (sensor should be connected, use a special intrusive clip for this purpose). Start the engine and adjust the oscilloscope signal until you have a clear reading on the display. You should see a square waveform similar to the shown in the introduction of the article, however, it’s advisable using the appropriate literature for comparison and diagnostics. 

> Ground test: enter in KOEO mode (Key On Engine Off) turn the digital multimeter dial to “voltage” test mode and place one probe tip on the CMP harness terminal corresponding to the supply voltage and the other on the terminal that should be the ground. Ideally, the voltage would be the same value as the previous test, if the difference is greater than 10% then you will need to check electrical resistance and continuity of the ground line from the CMP harness all the way down to the ECU.

4. Scan Tool Tests

Recommended tools: automotive scan tool with live data capabilities.

Optional tools: a professional automotive diagnostic scanner with live data graphing ability.

> Camshaft Position Sensor live test: using a scan tool for diagnosing a CMP sensor is highly dependant on your car manufacturer. This is due to the way scan tools read the information. Some manufacturers facilitate live data stream that “emulates” the oscilloscope wave pattern for the diagnostic but others don’t. Ideally, you should perform oscilloscope tests as the one explained in the prior section for a more precise diagnostic.


How to replace a bad Camshaft Position Sensor

The CMP sensor is usually located on the valve cover corresponding to engine bank #1 (the bank of cylinders where cylinder #1 is located).

> Preliminary steps: ensure the ignition switch is off and then disconnect the vehicle battery. Detaching the negative terminal is usually enough.

> Remove CMP sensor connector: this step may be tricky in some models. Generally speaking, the connector has a plastic tab that you should push down while you pull out the harness. But in many cases, there is a small security clip (sometimes plastic, sometimes metallic) that must be removed first using an adequate screwdriver.

> Remove CMP sensor: removing the CMP sensor is done by removing the clamping screw that holds it to the valve cover.

> Install the new CMP sensor: retire the old sensor and then install the new one. Double check that you’re using the adequate part number. 

> Reconnect sensor and battery: once you are done, reconnect the sensor (don’t forget security clips) and then the battery terminal.

> Clear DTC memory: using the OBD2 code reader clear the ECU memory to prevent any false code been stored. You will need entering in KOEO mode for doing so.

> Perform driving cycle: once you are done, drive the vehicle for more than 10 minutes. Try to accelerate as you would normally do. 

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