Updated at: 16-08-2022 - By: Lucas

While waiting for a green light at an intersection, have you ever seen a car’s speedometer start to whir? Bad speed sensors commonly cause problems like this. This type of issue can lead to more serious problems, so it’s critical that you get your car looked out. We’ll go over five symptoms that point to a faulty speed sensor in this post.

What Does A Speed Sensor Do?

Speed Sensor Symptoms

An engine control unit receives information about the vehicle’s speed via a speed sensor (ECU). Amount of gasoline pumped into your cylinders is then determined by the ECU for optimal performance.

Faulty wiring or, more likely, a malfunctioning speed sensor could be at blame if you’ve ever had a problem with acceleration.

How Does A Speedometer Work?

For the speedometer in your car, an electrical signal is sent by wires from the display panel to the instrument cluster, where you may see readings for things like fuel level, engine temperature, and speed.

This means that if your VSS is malfunctioning, you will likely see errors in your speedometer readings because the speedometer relies on information from the VSS.

What is the difference between input and output speed sensors?

Speed Sensor Symptoms-2

Both the input and output signals from the transmission are used to calculate the speedometer’s reading in automobiles. Speedometers display your speed based on an input signal created by sensors in either the flywheel or drive plate of the engine, and this signal is transmitted to either a mechanical or electronic device.

A vehicle speed sensor receives data from the final drive to power the speedometer (VSS). The ECU of your automobile receives information about your speed from a device called a vehicle speed sensor.

Only one of these systems can quickly become out of sync with each other when used to determine your vehicle’s exact speed. Faster or slower readings will be displayed depending on how closely the two signals are spaced apart.

Input Speed Sensor

There is a sensor that measures how quickly each wheel spins and then compares this value to engine rev per minute to ensure the speedometer is accurate (RPM). As a result, your ECU will try to bring both numbers closer together if they are out of sync.

Output Speed Sensor

The output speed sensor keeps track of the engine’s revolutions per minute (RPM) and relays that data to the transmission controller for gear selection. As an illustration, if you’re travelling at 40 mph and suddenly decide to accelerate, your engine revs will drop briefly while your wheels accelerate and the vehicle accelerates quicker.

The output speed sensor detects this brief drop in RPM when the wheel speed rises. There is no lag between throttle position and power being transferred to the wheels as a result of this information being sent to an ECU or TCM. Drivers who like to accelerate quickly from a stop may appreciate this feature.

The accuracy and dependability of the vehicle’s speedometer are enhanced by the integration of input and output speed sensors. Incorrect speedometer readings can be caused by a simple offset between the two signals, as previously discussed.

A sensor or wiring harness repair or replacement is likely if your car’s speedometer is malfunctioning. Even if you don’t feel confident doing it yourself, it may be less expensive to have them repaired than to have the entire assembly replaced.

Check your ABS system before purchasing an input or output sensor for your car, as this controls and checks their operating condition. A speed sensor checker will help you identify the problem.

Can a faulty output speed sensor prevent a vehicle from shifting?

It’s possible that a vehicle’s inability to shift is due to a bad speed sensor. A defective speed sensor could generate a difference between what computers are reading and what is actually happening mechanically, which is why this is critical.

You’ll notice the symptoms by being unable to choose gears, or if you shift up or down, just shifting to one gear.

The driver may believe their vehicle is experiencing something different from what mechanics view during an examination if their automobile thinks it’s in neutral even when it’s in gear.

What Are The Symptoms Of A Bad Speed Sensor?

Speed Sensor Symptoms-3

Five of the most prevalent signs of a faulty speed sensor are as follows:

Erratic or Inaccurate Speedometer Reading

One of the most prevalent symptoms of a faulty speed sensor is an inconsistent or erroneous reading on the speedometer. It’s possible that your speed sensor is malfunctioning if your speedometer jumps up and down or doesn’t display the right speed.

Hesitation When Accelerating

When you try to accelerate, your automobile may stutter because of a defective speed sensor. It’s critical to have your car examined by a mechanic if you experience this symptom because it could be caused by a variety of other issues.

Trouble Passing

Speed sensors that malfunction can make it difficult to overtake other vehicles on the highway. The ECU can’t figure out how much fuel to put in the cylinders if the sensor isn’t working properly. As a result of this, you won’t be able to pass other vehicles.

Check Engine Light Is On

You may have a speed sensor problem if your check engine light is on. Taking your car to a professional for a diagnostic test is the best way to discover the problem. A defective speed sensor could be responsible for your vehicle’s engine light.

Car Slows Down When Idle

There may be an issue with one or more of your car’s sensors if your vehicle slows down when it’s idling. Taking it to a mechanic for a diagnostic test is the best option for determining the source of the problem.

Having your car checked out by a mechanic if you’re experiencing any of these symptoms is critical. It’s recommended to have a malfunctioning speed sensor repaired as soon as possible because it might cause a slew of issues with your car.

Speed Sensor Troubleshooting

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Troubleshooting the problem of a broken speed sensor is not difficult. Tips for troubleshooting a defective speed sensor:

Check Wiring

Faulty wiring is one of the most prevalent causes of a speed sensor malfunction. You should initially check the wiring if you suspect that your speed sensor is defective. Doing so is as simple as ensuring all sensors are connected and working properly.

Clean Sensor

The sensors can malfunction if they become clogged with dirt and debris. If you suspect this is the source of your issues, you can try blowing compressed air or a brush on the sensor to see if it helps.

Replace Sensor

Speed sensor replacement may be necessary if you’ve tried everything else and nothing works. A few basic hand tools and an hour of work are all you’ll need to replace the sensor on your own.

To access the old sensor, you’ll need to remove some items from beneath the vehicle, but once you do that, replacing and removing the new one should be a breeze.

When all else fails, call a mechanic as soon as possible to take care of your vehicle’s issue.

Having an erroneous speedometer reading or improper acceleration might have serious effects, so you don’t want to wait too long to fix it.


What Causes A Bad Speed Sensor?

There are a number of possible causes for a malfunctioning speed sensor. In the majority of cases, the following are to blame:

  • Dirty or Damaged Wiring
  • Corroded Connections
  • Faulty Sensor

What Else Can Cause Erratic Speeds In My Car?

Other factors can contribute to your vehicle’s unpredictable speed. Here are a few examples:

  • Dirty or Faulty Air Filter
  • Bad Spark Plugs
  • Problems with the Ignition System

How Do I Know If My Speed Sensor Needs To Be Replaced?

The sensor should be replaced if troubleshooting has not resolved the issue. With a few simple hand tools, you should be able to accomplish this in most circumstances. The replacement should be completed in no more than an hour.

  • Inaccurate Speedometer Reading – Your speedometer won’t work properly if you don’t have a functional sensor. This could cause you to get into an accident, so it’s best to correct the problem as soon as possible!
  • Faulty Acceleration – If your car is hesitating or having trouble passing when accelerating, then this may be caused by faulty acceleration issues. You should try troubleshooting the issue and replacing any defective parts before taking action towards getting a new speed sensor installed.


If you see any of the signs of a malfunctioning speed sensor, you should have your vehicle inspected by a mechanic as soon as possible. It’s recommended to have a malfunctioning speed sensor repaired as soon as possible because it might cause a slew of issues with your car. Your car’s speed sensors are an essential component that must be maintained at all times!