Symptoms of a Bad Camshaft Position Sensor
What Happens When a Camshaft Position Sensor Wears Out
It can happen at any time without warning: You are driving on the highway, moving along in fast traffic, when your engine suddenly loses power. There is nothing to do but watch in horror as a vehicle approaching at 70 miles an hour rear-ends you.
Not a pretty picture, but it's already happened many times. Hopefully not to you.
The culprit behind some of these accidents is a failed camshaft position sensor (CMP sensor). Sometimes a CMP sensor can fail without warning, causing the engine to shut off. Other times, the driver may be unaware of developing symptoms until the engine refuses to start.
Here, we'll explore the symptoms of a bad camshaft position sensor and what you can do about it. But let's discuss first what the sensor does.
What Is a Camshaft Position Sensor?
Your engine's cylinder head houses one or two camshafts—a shaft equipped with offset lobes—to operate the intake and exhaust valves. The crankshaft, located in the engine block, drives the camshaft using gears, a timing chain, or a timing belt.
To determine which cylinder is in its power stroke, your car's computer monitors the rotating position of the camshaft relative to the crankshaft position using a camshaft position (CMP) sensor. It uses this information to adjust the spark timing and the operation of the fuel injectors. Thus, the CMP sensor affects fuel economy, emissions control, and engine efficiency.
The two most common camshaft sensors you'll see are the magnetic and Hall-effect types. Both transmit a voltage signal to an electronic control module or to the car's computer.
The magnetic type produces its own AC (alternate current) signal (a sine wave), and you can identify it by its two wires. The Hall-effect type uses an external power source to produce a digital signal (a "square wave," on-or-off) and has three wires.
Depending on the specific model of your car, your engine may have one or more cam sensors.
Symptoms of a Bad Camshaft Position Sensor
Just like every part or component in your car, the CMP sensor will eventually stop working when it's reached the end of its service life, because an internal part, wire, or related component has failed. The symptoms your engine may experience at this point can vary, depending on the type of sensor failure: for example, a problem in the circuit, the connector, the sensor itself, or a related component.
- On some vehicles, a failing camshaft sensor may lock the transmission in a single gear until you turn off and restart the engine. This cycle may repeat intermittently.
- If the sensor begins to fail while your car is moving, you may feel the car jerking while losing power.
- You may experience a noticeable loss of engine power. For example, the engine can't accelerate above 35mph.
- The engine may stall intermittently.
- You may notice poor engine performance including irregular acceleration, misfiring, hard starting, or surging.
- On some car models a failed CMP sensor will prevent ignition spark, so that the engine won't start at all.
Once your car's computer detects a CMP sensor failure, it will trigger the engine light and store a diagnostic trouble code (DTC) in its memory (see the table below for common camshaft position sensor trouble diagnostic codes). An intermittent or complete CMP sensor failure while on the road could be dangerous.
CMP Codes and What They Mean
Common CMP Trouble Codes
Source of Trouble
Circuit range or performance problem
Circuit low input
Circuit high input
Camshaft Position Sensor Location
As you may expect, the specific location of the camshaft position sensor varies by a vehicle's make and model. On most models you can find the sensor somewhere around the cylinder head. Look around the top section of the timing belt/chain cover (in the front of the engine) or at the rear end of the cylinder head. Some GM models may have a special compartment for the sensor.
Also, some Mercury Villager and Nissan Quest models locate the CMP sensor inside the distributor housing, as well as some Dodge Ram B1500, B2500, and B3500 series models with gasoline engines.
If necessary, check the vehicle service manual for your particular model. If you don't have the service or repair manual for your car, you may find a copy in the reference section of your local public library.
I highly recommend that you buy a relatively inexpensive, aftermarket repair manual for your specific vehicle make and model for reference when doing maintenance and small repairs.
CMP Sensor Troubleshooting
If your car computer has already triggered the engine light, you may retrieve the code (the DTC) using a code reader or a relatively inexpensive scan tool. If you don't own a code reader and can't afford to buy one, and you still can drive your car safely, just go to a nearby auto parts store that retrieves DTCs for free.
After confirming a CMP-sensor related trouble code, it's worth doing some simple tests. A trouble code pointing to a potential CMP sensor failure doesn't necessarily mean that the sensor itself is bad. You may be dealing with a wire, connector, or related component failure that you can fix yourself.
However, confirming the good or bad operation of a camshaft sensor may require a scope. A failing sensor signal, for example, may be hard to check without special equipment. Still, you can do some simple checks in your garage using a digital multimeter (DMM) tool.
- First, check the CMP sensor electrical connector and wires condition. Unplug the connector and check for rust or contamination, like oil, that is interfering with good electrical contact. Then check for wire damage: broken wires, loose wires, and signs of burns caused by nearby hot surfaces. Also, make sure the sensor wires are not touching spark plug wires or ignition coils, which may interfere with the sensor's signal.
- After these checks, use a digital multimeter that can test either alternate current (AC) voltage or direct current (DC) voltage, depending on your particular type of camshaft position sensor. You'll also need the correct electrical values for your particular type of sensor. You may find this information in your vehicle repair manual.
- With some sensors, you may back-probe the wires through the sensor electrical connector. If this isn't possible, see if you can unplug the sensor connector and attach a strand of copper wire to each terminal on the connector. Then, plug the connector back in so that the two strands stick out through the connector's housing. Another solution is to pierce through each wire using a pin, being careful not to short out the wires during your tests. If you use this last method, use electrical tape to cover the pin holes on the wires' insulation after you're done with your tests to prevent corrosion from creeping into the wires.
Testing a Two-Wire Sensor:
- If you have a two-wire, magnetic-type CMP sensor, set your multimeter to "AC volts."
- Have an assistant turn the ignition key on without starting the engine.
- Check for the presence of power flowing through the circuit. Touch one of your probes to ground (any metal part on the engine) and the other probe to each one of the sensor wires. If neither wire has current, there's a failure in the sensor's circuit.
- Have your assistant crank or start the engine.
- Touch one of your meter probes to either one of the sensor wires and the other probe to the other wire. Check your meter display and compare your reading to your manual specifications. In most cases, you'll see a fluctuating signal between 0.3 volts and 1 volt.
- If there's no signal, you have a bad CMP sensor.
Testing a Three-Wire Sensor:
- Once you identify the power, ground, and signal wires using your vehicle repair manual, test the sensor's circuit by setting your multimeter to "DC volts."
- Have an assistant turn the ignition key on, but don't start the engine.
- Touch the black probe on your meter to ground (a metal bracket, bolt, or metal surface on the engine itself) and the other probe to the power wire. Compare your reading to the specification in your manual.
- Have your assistant crank or start the engine.
- Touch the signal wire with the red probe from your meter and the ground wire with the black probe. Compare your reading to the specification in your vehicle repair manual. If the voltage signal is lower than the specification, or no signal comes out of the sensor, most likely the sensor is bad.
- Remove the sensor and inspect it for signs of physical damage or contamination.
Check the video below to see how you can perform these tests using a test light and a multimeter. It'll give you an idea of the nature of the tests too. If you can't find anything wrong with the sensor or its circuit, it's possible you may have an intermittent failure or a failure in a related component. For example, you may have a weakened or overstretched timing belt or timing-belt tensioner. A worn-out belt can prevent the camshaft and crankshaft from synchronizing, causing the CMP sensor to send the wrong signal.
Testing a Camshaft Position Sensor
Sensor Replacement and Cost
If you've confirmed that the camshaft position sensor is bad, you may want to replace it yourself. On some vehicle models, replacing the sensor is just as easy as unplugging the electrical connector, unscrewing the mounting bolt, pulling the sensor out, and installing a new one. On other models you may need to remove one or more components to gain access to the sensor (as you will see in the next video).
Check your car's repair manual for instructions on how to replace the sensor on your particular vehicle model.
Expect to spend anywhere between $30 and $100 (or more) for the sensor itself, depending on your vehicle model. If you take your vehicle to a car shop, you may be looking at $100 or more in labor expenses too.
Replacing a Camshaft Sensor (Nissan Altima)
The symptoms I discussed above are not clear signs that your CMP sensor is bad. Still, if you recognize one or more of these symptoms, try to diagnose the problem as soon as possible. The last thing you want is to get stuck in the middle of the road. Start by getting the trouble codes from your computer memory, and, if necessary, testing the sensor with the help of your vehicle service manual. Sometimes you can determine the cause of the problem and fix it yourself without spending too much time and money.
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