What Happens If A Transmission Speed Sensor Goes Bad?

A faulty transmission speed sensor can lead to numerous issues, including things like rough shifts, cruise control malfunctions, and a check engine light.

Most modern vehicles come with two transmission speed sensors, which are responsible for monitoring the input and output shaft speeds of the transmission. These sensors send information to your car's powertrain control module (PCM), which then compares actual gear ratios to desired ones. If one or both of these sensors fail, several issues can occur, including some that are pretty easy to spot.

One of the first and most obvious signs that you'll be able to detect is harsh shifting. See, the PCM relies on accurate data from the sensors to control gear changes. Without that data, shifts can become rough, delayed, or premature. This is a serious problem, and not just because it can cause an uncomfortable driving experience — automatic transmissions are designed to shift smoothly and at the right times, and harsh or poorly-timed engagement can cause damage to internal components, such as valve bodies, hydraulic lines, or even the gears themselves.

Another common symptom is cruise control failure or malfunction. Since the transmission speed sensors provide essential speed data, the PCM will disable cruise control if that data is unreliable. Furthermore, a failing speed sensor can cause your speedometer to act wild and unpredictably, too, which can make using cruise control tricky, if not outright dangerous.

Ivan Pantic/Getty Images

If you have a faulty transmission speed sensor, chances are it will illuminate the check engine light (CEL) on your dashboard. While this is a common symptom of a bad transmission speed sensor, the light itself isn't a diagnosis, as many things can cause it to appear. That said, you don't have to be a mechanic to outsmart your check engine light — all you need is a basic OBD-II code reader. Then, you can identify the code that triggered the light, which will point you in the direction of the deeper problem.

As for replacing a faulty transmission sensor, whether it is a DIY job or not depends on your car's transmission and your abilities. Most vehicles have sensors that are relatively easy to reach, meaning you can generally remove and replace them without specialized tools. In these cases, most home mechanics and anyone diligent enough to follow a handy YouTube tutorial should be able to tackle the replacement. However, depending on your vehicle, the sensors may be harder to access. If you're inexperienced or are struggling to identify the sensor, it's probably best to leave your replacement to a professional.

So, while driving with a faulty transmission speed sensor may technically be possible, you risk damaging internal components, which can cost an awful lot more than a simple sensor replacement. Addressing the issue promptly will protect both your wallet and your transmission's health in the long run.