Symptoms of a Bad Crankshaft Position Sensor

Your car contains a large variety of sensors on board, and one of these is called the crankshaft position sensor, or CKP for short. Like any car part, the crankshaft position sensor can fail like anything else, and when it does fail, it can cause various problems.

It can be replaced for anywhere from a few dollars for some models, to a more common price range of $40-$150. This article will explain what symptoms the crankshaft position sensor may create, how to diagnose it, and what could happen if you allow it to go ignored.

What Does a Crankshaft Position Sensor Do?

The crankshaft is the rotating assembly that is inside the engine that connects the crankshaft, connecting rods, and pistons. When the engine is running, the crankshaft spins as the pistons move in a linear motion, thus creating the rotational force that is required to propel the vehicle.

As you may guess, the CKP sensor monitors the engine’s crankshaft speed and position. The sensor generates a signal, through a toothed or notched wheel that is usually on the crankshaft or harmonic balancer.

Therefore, the crankshaft position sensor is often located at the front bottom of the engine, although again, it may differ from vehicle to vehicle.

The engine computer, often called the powertrain control module (PCM) uses the CKP sensor signal when computing spark timing and (for most vehicles) fuel injector synchronization.

In addition to spark timing and fuel injector synchronization, the PCM uses the CKP sensor signal when controlling other functions, like the variable valve timing system (if applicable). For all vehicles made from 1996 onward, the PCM uses the CKP sensor to determine whether the engine has misfired. 

Bad Crankshaft Position Sensor Symptoms

The symptoms of a bad or failing crankshaft position sensor might initially seem intermittent. Sometimes, they can be confused with another engine fault. However, the symptoms become more obvious as the crankshaft position sensor continues to fail.

However, a bad or failing crankshaft position sensor will manifest the following:

#1. Hard Starting with a Rough Idle.

Symptoms of a failing or bad crankshaft position sensor often start out as the engine being hard to start and then running with a rough idle. This is linked to the ECU not having the correct information to synch fuel and spark delivery to the cylinders precisely.

Early on, you might not mentally connect the hard starting and rough idle to a failing crankshaft position sensor.

There are a lot of other mechanical faults in an engine that can do the same thing. However, as the idle worsens and the engine labors for longer when starting, you should become increasingly suspicious that the crankshaft position sensor is starting to fail.

#2. Intermittent Stalling.

Many cars with a failing crankshaft position sensor also experience seemingly random stalling. One minute, the sensor performs normally, but it fails or sends intermittent pulses, which confuses the ECU.

Unfortunately, these stalls often happen when you’re accelerating hard, which can make merging into traffic at highway speed difficult. This is usually the point where many people take action after an unexpected white-knuckle experience on an on-ramp.

#3. Inconsistent Acceleration.

When the crankshaft position sensor fails, inconsistent acceleration and intermittent stalling often occur in tandem. You might step on the accelerator, and the car surges, only to suddenly sputter.

If the electric pulses from the failing crankshaft position sensor aren’t transmitted for too long, the internal combustion process in the cylinders stops, and the car stalls. If the pulses are sporadic, the engine surges again, leaving you feeling like it might die at any moment.

#4. The Engine Won’t Start.

When a crankshaft position sensor goes from failing to bad, it will cause the engine to not start or become severely flooded. As the failing crankshaft position sensor worsens, there is an imbalance between the timing of the fuel delivery and the spark.

This type of accidental flooding or timing failure is often the next step after a hard starting, rough idle.

#5. Poor Gas Mileage.

Unburned fuel and poor fuel injector performance caused by a failing crankshaft position sensor can cumulatively affect your engine’s fuel consumption, leading to poor gas mileage.

You might not notice a major dip in MPG at first, but the ECU monitors will start to nosedive as the sensor continues to fail the mile-to-the-gallon rating. Unburned fuel entering the exhaust system can also cause problems with the catalytic converter and foul-smelling exhaust.

#6. Engine Misfires.

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The fuel injector and spark plug performance inconsistencies caused by a failing crankshaft position sensor can also leave unburned fuel in the cylinders, leading to recurring misfires.

This is a symptom of a bad crankshaft position sensor that occurs as stalling and inconsistent acceleration problems worsen, especially when you need to step down hard on the accelerator.

#7. Foul Smelling Exhaust.

Anytime fuel goes unburned due to a failing crankshaft position sensor, it can travel into the exhaust system, causing foul-smelling exhaust.

The odors might be reminiscent of sulfur or rotten eggs or smell immediately of evaporating gasoline. Usually, these odors happen after a bad crankshaft position sensor causes misfires from poorly timed fuel injectors.

#8. Engine Vibrations.

Engine vibrations from a failing crankshaft position sensor can often cause a series of small misfires, often occurring in tandem with a rough idle.

This is more likely to happen when you’re sitting at a red light with the engine running hot. However, you might also notice it when you step down hard to accelerate up to highway speed.

#9. Check Engine Light.

A failing crankshaft position sensor will trip the check engine light eventually. It’s just a matter of where the automaker set the threshold for a problem. Usually, this starts as a flashing check engine light that happens when the engine misfires or stalls. Sometimes, the light will simply stay on.

The ECU will throw a code that you can read on an OBD II code reader anytime it does this. Usually, you’ll get a P0301 to P0308 code, which indicates the cylinder or cylinders that had a problem. You might even get a P0420 code, which indicates a general problem with the catalytic converter.

Though the codes that you need to pay attention to the most with a bad crankshaft position sensor are P0335 (Crankshaft) or P0340 (Camshaft), this might also include a code P0325, which indicates a Knock Sensor Malfunction.

What Caused A Crankshaft Sensor To Fail?

There are a number of reasons that can cause a crankshaft sensor to fail

For crankshaft sensors that contain magnets, often dirt can build up on the magnet, which can obstruct the sensor and prevent it from seeing the spin of the engine.

Another factor of the life cycle of a crankshaft sensor is the age of the sensor, as the magnet itself will lose its pull and become less able to provide accurate signals to the ECU. Worn, loose or damaged wires can influence the sensor’s ability to provide accurate signals to the ECU.

While a crankshaft sensor is quite clever, there are ways it can be tricked. If the gear that the sensor is observing suffers damage then the sensor can’t be sure of what it is observing which leads to confusion in sending signals to the ECU. This will bring up an engine check light.

It is also possible the sensor was damaged through travel by the crankshaft pickup, or even by road debris. If the component is repaired but the engine is still having difficult starting, that unnoticed damage to the sensor could be the culprit.

How To Know If Your Crankshaft Is Bad

If you have a no-start problem, and the ignition is just clicking over but your engine refuses to fire, you may have a bad crankshaft position sensor.

With no crankshaft position sensor the engine will not know when to fire the spark plugs or even inject fuel! So don’t plan on starting it any time soon!

How To Check the Crankshaft Position Sensor

If you can find your crankshaft position sensor under your hood, the most common spot is either on the front or back of the crankshaft on a bell housing, or behind something. You can try to test it to see if you can blame your no-start on this sensor:

• Check it visually: Look for a damaged housing as it may have been burnt, or it probably has dirt/oil/debris and has stopped working properly.
• Check it with a multimeter: If you have a multimeter handy, pull it out, set it to ohms, and read the measurement on the crankshaft position sensor.
• Interpret your readings: The meter will read a resistance value of anything between 200 and 1,000 ohms (or more-depending on the model). If it reads 0 ohms, it needs to be replaced.

How to Replace Your Crankshaft Position Sensor

You can tackle replacing your own crankshaft position sensor and save yourself $100 or more, which is the labor time to have a shop do it. You should have decent mechanical skills and an adequate automotive tool kit.

Once you have identified the crankshaft position sensor, you can go through the steps below to replace a failing crankshaft position sensor.

• Step One – Disconnect the negative terminal on the vehicle’s battery.
• Step Two – You will need to gain access to the crankshaft position sensor. Some ancillary items that may or may not need removing will be: the timing cover; the serpentine belt and the crankshaft pulley (most of the time it is at the front of the engine); the starter motor (if near the transmission).
• Step Three – Remember the 3 wires attached to the crankshaft position sensor? Take a picture of them with your phone or label them in a side drawing. They are an input, output, and ground wire. Carefully set them aside without letting them make contact with any wet or metallic parts.
• Step Four – Remove the 2 small bolts to release the crankshaft position sensor from the engine.
• Step Five – Compare the failed crankshaft position sensor and the new one to ensure they match.
• Step Six – Reattach the 3 wires to the sensor in the same ports (this is critical). If you do not connect to the port correctly, the new sensor will not work, and you will be exactly back to where you started!
• Step Seven – Reattach the new crankshaft position sensor by connecting the original bolts that held the original sensor to the engine. Reattach everything that was removed in reverse order to the disassembly process. Finally, reconnect the vehicle’s battery.
• Step Eight – Clear all the codes, and start the vehicle. Determine how the vehicle is idling & rev the engine a little, and see if you can duplicate the symptoms of a bad crankshaft position sensor.

If the vehicle seems to operate normally, take the vehicle for a 15–20-minute test drive. You ideally want to drive on the town roads and residential streets, to ensure that there are no stalling, misfiring, or surging acceleration troubles.

Then you can take it to the highway to see how it accelerates when merging and passing at high speed.

Cost of Crankshaft Position Sensor Replacement

The cost of replacing a crankshaft position sensor is usually specific to the vehicle, but can range considerably due to the cost of parts and labor involved. If someone is mechanically inclined and owns their own basic set of hand tools, they may be able to do this work themselves and save some money.

If you need a crankshaft position sensor replaced by a mechanic, you can expect to pay anywhere from $175 to $300.

The cost of the sensor Themselves range from $45-$85 and illustrates the potential savings achieved by doing this repair yourself.

Common Causes of Crankshaft Position Sensor Failure

There are several different ways that a crankshaft position sensor can fail or various reasons why it may be damaged, with some being more common than others.

The most common reason for the failure of a crankshaft position sensor is damage to the circuitry, which is generally in the form of actual damage to the sensor’s plugs, or causing the sensor to lose continuity. Crankshaft position sensors can fail internally or due to a pinched or chafed wiring harness or pigtail.

It’s also not uncommon, for crankshaft position sensors to be damaged from debris when it gets struck from the undercarriage.

Due to the fact that the crank position sensor is located relative to the lower half of a vehicle’s engine, tire casings, and other rubbish on the road become serious adversaries to the crank position sensor when they strike the crank position sensor while in the lane of travel.

Age and heat are also deadly foes of a vehicle’s crankshaft position sensor. Given enough time, the actual crankshaft position sensor ‘body’ can become brittle making it susceptible to failure under vibration. Unfortunately, there is not much that can be done to prevent this from happening.

Diagnosing A Failing Sensor

One of the easiest ways of diagnosing a failing crankshaft position sensor is with a multimeter. The resistance should be checked across this sensor, and then compared to resistance readings provided by the vehicle manufacturer.

Infinite resistance across the terminals of the sensor would indicate an “open” condition, while nil recordable resistance would indicate a “shorted” condition. Output of the crankshaft position sensor can also usually be verified while cranking a vehicle’s engine, in the form of AC millivolts. 

Alternatively, a quality OBD-II scan tool can also be used to verify crankshaft position sensor feedback. The live data reporting via the scan tool should provide either a rise/fall in pulse counts that mirror associated rises or falls in engine RPM.

If there is no associated feedback, further diagnosing of the crankshaft position sensor, and the circuit, would be justified.

Can I Drive with A Bad Crankshaft Position Sensor?

Driving with a known faulty crankshaft position sensor is never advisable due to the risk of experiencing drivability issues.

Some engines will indeed run with a faulty crank sensor, but others will not, and even ones that do will not necessarily do so reliably.

Modern computer-controlled engines depend on accurate feedback related to engine timing data to operate combustion efficiently and consistently. Thus, if there is not a constant stream of data, a vehicle’s engine may run erratically at best.

In some instances, an engine can stall and refuse to restart if the signal from the crankshaft position sensor is interrupted. 

For these reasons, crankshaft position sensor related vehicle issues should always be diagnosed and repaired immediately. Doing so can potentially save you from being stranded on the side of the road, and will hopefully remove any doubt.

If you do not feel comfortable, or competent to make this repair on your own, than an appointment should be scheduled with a reputable automotive service center at your earliest convenience.

FAQs.