7 Symptoms of a Bad Speed Sensor (and Replacement Cost)

(Updated on March 22, 2021)

On modern fuel injected engines, there are many sensors that work together to tell the engine or transmission computer the state of the world. The computer needs to know how fast the wheels are turning, how fast the engine is spinning, and make sure that all monitored parameters remain within specifications.

A “speed sensor” could refer to many different types of sensors. Some cars have wheel speed sensors, most cars have engine and transmission speed sensors, and some have all of the above.

Keep reading to learn about the various speed sensors, symptoms of a malfunction, and their replacement cost.

What is an Engine Speed Sensor?

bad crankshaft position sensor

There are many different types of engine speed sensors, but they all achieve the same goal: letting the ECU know where the crankshaft and camshaft(s) are at all times.

An engine speed sensor may also be referred to as the crankshaft position sensor, the camshaft position sensor, the crankshaft/camshaft angle sensor (CAS), or the engine position sensor.

The timing of the engine is critical for proper combustion. The ECU needs to know precisely when to inject fuel, open the valves, and ignite the air fuel mixture for optimal combustion. If this process isn’t synchronized perfectly, the engine will run poorly, if it runs at all.

Depending on the engine implementation, your car may have a separate sensor for the crankshaft position and camshaft position, or just one sensor to perform both jobs.

Crankshaft Position Sensor Types

crankshaft position sensor replacement cost

There are several common types of crankshaft position sensors. These sensors typically use an electromagnetic or optical sensor that does not require contact between moving parts. Without contact between moving parts, these sensors usually have a long life, because they are not subjected to wear.

Variable Reluctance Sensor

Variable Reluctance sensors, also called variable reluctor, VR, or magnetic sensors, are similar to pickups you might find in an electric guitar or bass. They use a magnet with a coil of wire wrapped around it.

Variable reluctance sensors output a voltage when a ferrous metal on a toothed trigger wheel passes by the sensor, creating a current on the coil of wire inside the sensor. These sensors produce a sine wave signal as the metal approaches the coil, then moves away.

Hall Effect Sensor

Miata CAS

A Hall Effect sensor (sometimes called a Hall sensor) is also magnetic. The main difference between a Hall Effect sensor and a variable reluctance sensor is that a Hall Effect sensor is digital, where the VR sensor is analog. This means it produces a square wave and can detect metal in front of the sensor, even when that metal isn’t moving.

The crankshaft angle sensor (CAS) on early 1.8L Mazda Miatas is a good example of a Hall Effect sensor. This sensor performed double duty; since early Miatas lacked variable valve timing, this one sensor could tell the ECU where both the crankshaft and camshaft were at any given time.

Optical Sensor

An optical sensor uses light that shines through a spinning disk to determine speed. Like the Hall Effect sensor, an optical sensor also produces a square waveform.

First generation 1.6L Miatas used an optical CAS sensor instead of a Hall Effect sensor.

What is a Transmission Speed Sensor?

transmission speed sensor

The transmission speed sensor talks to the ECU or TCU to help an automatic transmission know which gear it should be in. There are two main types of transmission speed sensors: the input shaft speed sensor and output shaft speed sensor.

Input Shaft Speed Sensor

An input shaft speed sensor measures the speed of the input shaft and torque converter, which matches the speed of the engine.

Output Shaft Speed Sensor

The output shaft speed sensor monitors the speed at the back of the transmission, after the transmission’s gear ratio has already been applied. This may be used by the speedometer or odometer to determine a vehicle’s ground speed.

What is a Wheel Speed Sensor?

wheel speed sensor

Wheel speed sensors may perform several jobs, including ABS function, traction control, stability control, ground speed (your speedometer and odometer), and tire pressure monitoring.

The ECU monitors all wheel speed sensors at the same time. If the ECU notices a difference in one or more wheel speed sensors, it can use this data to determine what’s going on in the environment.

Some vehicles are programmed to respond defensively to this data. For instance, most modern cars will brake a wheel that is spinning much faster than the other wheels, because it’s obvious to the computer that the fast spinning wheel lacks the necessary traction to propel the vehicle forward.

Modern traction control systems are very similar to traditional limited slip differentials, increasing the amount of torque that can be applied to the other wheel(s) on the other side of an open differential.

Some wheel speed sensors are built into the hub assembly, while others are standalone sensors that are easier to change.

Bad Engine Speed Sensor Symptoms

1) Startup Problems

trouble starting car

Engine speed sensors are often a critical component of a vehicle’s timing. If the ECU doesn’t know the position of the crankshaft or camshafts, it won’t know when to fire the spark plugs.

A bad crankshaft or camshaft position sensor may prevent the car from starting.

2) Check Engine Light

check engine light

Some vehicles have redundant sensors that allow the vehicle to start and run even if one speed sensor is bad. In these cases, you are likely to see a check engine light with a code that should give you a clue which speed sensor is faulty.

Check engine lights can be read for free at most automotive parts stores, but you can also find an inexpensive code scanner to use at home if you wish.

Bad Transmission Speed Sensor Symptoms

1) Broken Speedometer or Odometer

how many miles can a car last

Many speedometers are powered by speed sensor connected to the transmission. If this speed sensor fails, your speedometer may not work. Some odometers use the same sensor as the speedometer, and may cease to function as well.

2) Slow or Harsh Shifting

dual clutch transmission paddle shifters

Without a speed sensor, the transmission control unit (TCU) may have a hard time knowing when or how quickly to shift gears. You may experience rough shifting, or no shifts at all.

3) No Cruise Control

Cruise control relies on the knowledge of a vehicle’s speed to function correctly. If a speed sensor fails, you may not have cruise control available until the sensor is fixed.

Bad Wheel Speed Sensor Symptoms

1) ABS Light

abs light comes on

On vehicles with ABS, a bad wheel speed sensor will trigger an ABS light. Most ABS systems cannot function properly without a good signal from all four wheel speed sensors.

If you see the ABS light illuminated, don’t expect your ABS system to help you out if you need to brake hard. It’s best to address this issue right away so you can retain full control over the vehicle under heavy braking.

2) Traction or Stability Control Light

traction control light on

In addition to the ABS light, you may also see traction and stability control lights illuminate. Without good information from each wheel, the ECU may not have enough data to know the relative speed between the driven wheels.

Speed Sensor Replacement Cost

Looking for Replacement Parts?
We recommend Parts Geek or Advance Auto Parts for the best prices and selection.

Engine and Transmission Speed Sensors

Engine and transmission speed sensors vary in cost depending on the type of sensor. A crankshaft or camshaft position sensor could cost anywhere from $15 to $250. Aftermarket parts tend to be cheaper. Transmission speed sensors tend to cost around the same.

Labor varies depending on the engine type, as some sensors are more accessible than others. If your mechanic needs to drop the transmission to replace a transmission sensor, you’re looking at bill around $800-1,000.

Wheel Speed Sensors

Wheel speed sensors are generally pretty accessible and easy to replace. These sensors can be purchased relatively inexpensively, between $15 and $150 for the part alone. Fortunately, most vehicles tend to fall toward the lower end of that scale.

If the ABS sensor is integrated with the hub assembly, expect to pay a bit more, both for the entire hub assembly and the installation labor. A hub assembly will cost about $200, and the labor should run you about $200.

14 thoughts on “7 Symptoms of a Bad Speed Sensor (and Replacement Cost)”

  1. I need help,my Lexus is250 speedometer is not working perfectly and now the car won’t start, please can someone tell me what to do to make the car start, do I need to change the speedometer before it will work?

    Reply
    • Hello Shaffer, I would start by having the battery tested. I am skeptical that a speedometer failure would prevent your car from starting.

      Reply
  2. The battery has been tested already but still the car won’t start and is not showing the necessary light on the speedometer and right from time the speedo mileage is not working, i need help what should i do?

    Reply
    • I would take your car to a qualified mechanic for an in-person diagnosis. This type of thing is pretty tricky to diagnose over the internet.

      Reply
  3. Buick 2003 3.1 century has a long shift from 1st to 2nd gear after starting the car. No problem putting it in gear (drive) . It will take about 10 minutes before it get s into 2nd with s hard shift then holds a while then 3rd afterwards it’s smooth sailing and warmed up by then but once stopped and parked for a while it starts over again unless it’s not parked long.

    Reply
  4. I have a 1990 c2500 the shift timing is off and I’m getting a code of speed sensor fault. Yesterday in the middle of driving it wouldnt shift out of 1sst gear. Reverse works. But I have noticed emissions issues and improper shift timing. No slippinng,it just wont shift.

    Reply
  5. I have a 2004 Chrysler Pacifica I find myself driving then car goes into auto stick and it drives roughly and don’t pick up speed could you tell me the problem I was thinking speed sensor

    Reply
  6. Hie, I have a Toyota Sienta Cvt automatic transmission. It just failed to go foward whilst I was driving. I diagnosed the problem and found that it had damaged the drive seal and the clutches. I have replaced the drive seal and clutches and the car now has forward drive but the problem is that it won’t shifting into high gears. What could be the problem causing it to behave in that way? ?

    Reply
  7. I bashed up over a curb the other day. This morning the speedometer isn’t responding, the car won’t shift out of, I think it’s second, it’s max speed is 24mph according to my cell phone, and the power steering has disengaged. It was interesting to read that there’s a sensor down near the wheel because that’s what got knocked around under the tire, you could feel it amidst the jostle. I’m taking it to an automechanic tomorrow morning but are these symptoms generally consistent with a faulty sensor or do you think the steering and transmission are related to something else? I was thinking, it seems like it could be a safety feature to prevent people from getting away with driving without the governor engaged in assessing speed – like preventing a loophole for people to get away with driving without having a governor. Is that what’s probably happening or is it more likely multiple issues? I am driving a 2006 Saturn Ion that has been corrected for all of the recalls.

    Reply
  8. I have a unique problem.. I have a 2016, 370z , 6 peed , manual.. that won’t cut off.. If I start it and don’t put the car in motion… It shuts off just fine every time… but any motion.. whether in gear or just in neutral drifting down a hill … triggers something… and it then won’t shut off.. any suggestion would be appreciated…

    Reply
    • When you say “it won’t shut off”, do you mean you can turn the ignition to OFF, remove the key, and the car stays running? Sounds like the ignition may be bad.

      Reply

Leave a Comment