How do I know which camshaft sensor is bad?

Determining which camshaft position sensor is failing starts with checking diagnostic codes, then testing the sensor(s) and their wiring. In engines with more than one camshaft position sensor, you’ll compare readings and signals to identify the faulty unit.

What a camshaft position sensor does

The camshaft position sensor monitors the position and speed of the camshaft(s) so the engine control module can time fuel injection and ignition. There are two common types: Hall-effect sensors that produce a digital switching signal and variable reluctance (VR) sensors that generate a small AC voltage. The sensor typically requires a 5-volt reference, a ground, and a signal wire, all sent to the ECM through the wiring harness.

Common symptoms of a failing camshaft sensor

Signal loss or degradation from the sensor can cause a range of driveability issues. The most telling symptoms involve the engine management system failing to synchronize timing properly.

• Check Engine Light with camshaft-related codes such as P0340, P0341, or similar P-codes


• Misfires, rough idle, or reduced power under load


• Difficulty starting or a no-start condition


• Engine stalls or shuts off unexpectedly


• Intermittent or erratic performance, especially at higher RPMs


• Limp mode activation in some vehicles

These symptoms strongly suggest a camshaft sensor issue, but they can also be caused by wiring problems or crankshaft sensor faults. A methodical test plan is needed to confirm the culprit.

Diagnostic approach to identify the faulty sensor

Use a structured approach that combines codes, wiring checks, live data, and sensor testing. The exact steps depend on whether your engine has a single camshaft sensor or multiple sensors.

Single camshaft sensor engines

For engines with one camshaft position sensor, follow these steps to confirm whether the sensor itself is bad or if another system is at fault.

• Use an OBD-II scanner to pull camshaft-related codes and view freeze-frame data for context on operating conditions.


• Visually inspect the CMP connector and wiring for oil intrusion, corrosion, damaged insulation, or loose connections.


• With the ignition ON (engine OFF), measure the sensor’s power and ground at the connector using a multimeter to confirm a stable 5V reference and a solid ground.


• Check the signal line by observing the sensor’s waveform with an oscilloscope or a data-stream view in a capable scan tool while cranking or running. A clean, repeating waveform is expected; a flat or distorted signal indicates a faulty sensor or wiring issue.


• Verify the mechanical timing alignment if there is any chance the timing belt/chain has jumped or stretched, which can alter the sensor’s timing reference.


• If available, perform a swap test by replacing the sensor with a known-good unit or swapping with a spare to see if symptoms resolve.

When the sensor’s replacement resolves the issue, the CMP was faulty, and the diagnosis is confirmed. If problems persist after replacing the CMP, continue investigating other possible causes such as the crankshaft position sensor, wiring harness, or ECM.

Dual camshaft sensor engines

Some engines use two camshaft position sensors—one on each bank or on each cam. In these setups, you’ll compare signals and codes for both sensors to isolate the failing unit.

• Check for bank-specific or sensor-specific codes (e.g., P0340/P0341 for one sensor and bank-specific variants for the other). Note the exact code and bank in the data display if possible.


• View live data for both sensors and compare their timing relative to the crankshaft sensor. If one sensor’s signal is missing, erratic, or out of phase, that sensor is suspect.


• Use a scope or high-quality data stream to compare both sensor waveforms while cranking and running. A healthy system shows consistent, regular timing signals from both sensors that align with crank data.


• Perform a controlled swap test where feasible: replace the suspect sensor or swap sensor positions (if service manuals allow) to see if the fault follows the sensor.


• Inspect both sensor harnesses for damage, and check the timing mechanism (belt/chain, tensioners) for wear or misalignment, which can affect both sensors’ performance.

In dual-sensor systems, a faulty reading from one sensor often leaves the other functioning, but timing correlation between the two is critical. Correcting or replacing the bad sensor usually restores proper operation.

Additional checks and practical tips

Beyond testing the sensors themselves, a few practical steps help ensure an accurate diagnosis and a lasting repair.

• Keep the work area clean and avoid contaminating sensor connectors with oil, coolant, or dirt during inspection and testing.


• Use the vehicle’s service manual to confirm exact sensor types, pinouts, and test procedures for your make and model.


• Remember safety: disconnect the battery before disconnecting electrical connectors or probing circuits to avoid shorts or shock.


• Consider common failure points like aging wiring insulation, brittle connectors, or exposed wiring near the exhaust manifold.

If you’re not comfortable performing electrical and timing work, seek a qualified technician. Incorrect wiring or timing adjustments can cause costly engine damage.

What to do next: replacement and sourcing

If a CMP is confirmed to be faulty, replace it with the manufacturer-recommended part to ensure proper timing and compatibility. Always use OEM or reputable aftermarket parts and follow torque specifications and sensor installation procedures from the service manual.

• Obtain the correct sensor for your engine’s exact model, year, and VIN, especially for dual-sensor setups or variable valve timing systems.


• Replace the sensor O-ring or gasket if the sensor housing requires it, and ensure the connector seals are intact to prevent future corrosion.


• After replacement, clear any codes, perform a road test, and re-scan to confirm the fault is resolved and no new codes appear.

Ongoing driveability issues after a new sensor can indicate secondary problems such as timing misalignment, crank sensor faults, or ECM software needs. If problems persist, consult a professional for a deeper diagnosis.

Summary

Pinpointing a bad camshaft position sensor involves a mix of code reading, electrical testing, live data observation, and, when possible, sensor swapping. Engines with multiple CMPs require careful comparison between sensors and timing data to identify the faulty unit. Always start with a clean diagnostic path, verify wiring and timing, and replace with quality parts while following the vehicle’s service manual guidelines. When in doubt, seek professional help to avoid misdiagnosis or potential engine damage.

How do I know which crankshaft sensor is bad?

You can tell a crankshaft sensor is bad by using an OBD-II scanner to check for trouble codes like P0335, and by observing symptoms such as the Check Engine Light illuminating, difficulty starting or engine stalling, rough idling, misfires, and a noticeable drop in fuel efficiency. A physical inspection and a multimeter test can also help confirm the issue by checking the sensor's resistance and wiring for damage.
 
You can also check out this video:1mAuto Fix DiaryYouTube · Mar 6, 2025
Symptoms of a bad crankshaft sensor

• Check Engine Light: This is one of the most common indicators. An OBD-II scanner can confirm the problem by reading specific codes. 
• Difficulty starting: The engine may crank longer than usual or fail to start at all. 
• Stalling: The engine might suddenly shut off while driving or idling. 
• Rough running: The engine may run rough, misfire, or vibrate, especially at idle or during acceleration. 
• Poor performance: You may experience reduced acceleration and decreased fuel efficiency. 

How to test the sensor

1. Scan for codes: Use an OBD-II scanner to check for error codes such as P0335 (circuit malfunction) or P0336 (range/performance). 
2. Inspect for physical damage: Check the sensor and its wiring for any signs of cracks, fraying, or other damage. Look for loose or corroded electrical connectors. 
3. Test the resistance: Use a multimeter to measure the sensor's resistance. You will need to disconnect the sensor, set your meter to the resistance setting (ohms), and compare the reading to the manufacturer's specifications. A helper may be needed to crank the engine while you test. 
4. Check the signal (if possible): For some sensors, a helper can crank the engine while you use a multimeter or oscilloscope to monitor the sensor's voltage signal. The reading should be consistent. 

Which camshaft position sensor is bank 1?

Bank 1 refers to the side of the engine where the number one cylinder is located. To find the camshaft sensor for bank 1, first, identify the #1 cylinder, and then locate the sensor on the camshaft on that same side of the engine. For V-type engines, bank 1 is one side, and bank 2 is the other; for inline or four-cylinder engines, there is only one bank and the designation isn't relevant. 
This video explains how to locate engine bank sensors: 1mYOUCANICYouTube · Feb 14, 2023
How to identify bank 1

• Locate cylinder #1: The first step is to find the #1 cylinder in your engine. You can usually find this by looking for the cylinder closest to the front of the engine, or the cylinder number plate. 
• Determine the correct bank: The bank with cylinder #1 is always bank 1. 
• Identify the camshaft sensor(s): Once you've identified the bank, find the camshaft position sensor(s) on that bank. 
• Consider sensor types: If there are two sensors on bank 1, they are usually labeled as Sensor A (intake) and Sensor B (exhaust). 

This video explains how to locate engine bank sensors, including Sensor A and Sensor B: 59sHVAC Mechanic YouTube · Nov 3, 2024
What to do if you have a code

• Use the code to pinpoint the issue: A diagnostic trouble code (DTC) like "P0340" will indicate which sensor needs to be replaced. 
• Check wiring: After finding the sensor, inspect its wiring and connector for damage or corrosion. 
• Test the sensor: You can test the sensor's output signal to see if it is faulty. 
• Replace the sensor: Replace the sensor if it is faulty to restore proper engine timing and function. 

How do I test my camshaft position sensor?

To test a camshaft position sensor, use a multimeter to check for the correct voltage supply and ground, and then test the signal wire by watching for voltage fluctuations as you bring a metal object near the sensor. If the voltage drops to zero when metal is near and returns to normal when it's removed, the sensor is likely working correctly. If the voltage remains constant, the sensor may be faulty.
 
Before you begin

• Ensure the vehicle's ignition is on, but the engine is not running, unless instructed to crank the engine.
• Consult your vehicle's repair manual to identify the correct wires for the ground, power, and signal for your specific model, as they can vary.
• Park the vehicle on a level surface, engage the emergency brake, and shift to park or neutral. 

1. Check power and ground connections 

• Set your multimeter to the DC volts setting (20V is a good range). 
• Identify the power and ground wires using the vehicle's wiring diagram. 
• To check the ground: Connect the black probe to the ground terminal of the sensor connector and the red probe to the positive battery terminal. You should see battery voltage (around 12.612.612.6 volts). 
• To check the power: Connect the red probe to the power (usually around 555V or 121212V) terminal and the black probe to the ground terminal. You should see the expected voltage reading, for example, 555V. 

This video demonstrates how to test the ground and power connections of a camshaft position sensor: 55sAuto Repair GuysYouTube · May 17, 2021
2. Test the signal wire 

• Ensure the ignition is still on. 
• Set your multimeter to measure DC volts. 
• Connect the black probe of the multimeter to the sensor's ground wire and the red probe to the signal wire. 
• Turn the engine to the "crank" position, and check for a voltage signal output. 
• While the engine is cranking, observe the voltage on the signal wire. It should fluctuate. 
• Another method: With the key in the "on" position, watch the signal wire voltage. When you bring a metal object close to the sensor, the voltage should drop to zero. When you move the object away, the voltage should return to the normal reading (often around 555V). 

You can watch this video to see how to test the signal wire with a multimeter: 47sHVAC Mechanic YouTube · Jun 18, 2025
3. Interpret the results

• If the voltage drops to zero when a metal object is near and returns when it's removed, the sensor is likely functioning correctly. 
• If the voltage remains constant (either high or low) and does not fluctuate, the sensor is likely faulty and may need to be replaced. 
• If the sensor shows no signal, the issue could be with a fuse, the wiring harness, or the powertrain control module (PCM). 

What damage can a bad camshaft sensor cause?

Poor Acceleration and Performance: With incorrect data feeding into the engine system, you might notice sluggish acceleration or reduced power during drives. Reduced Fuel Efficiency: Faulty sensors can lead to inefficient fuel combustion, resulting in poor mileage and higher costs at the pump.