Common and Relatively Easy to Check:
* Speed sensor: This is the most common culprit. The vehicle speed sensor (VSS) measures the rotational speed of the output shaft of the transmission and sends that signal to the instrument cluster. A faulty sensor will prevent the speedometer from working, and may also affect the odometer, cruise control, and possibly the transmission's shift points. This sensor is relatively inexpensive and easy to replace, but its location varies depending on the transmission type.
* Fuse: Check your owner's manual for the fuse related to the instrument cluster or speedometer. A blown fuse is a simple and inexpensive fix.
* Wiring: Check the wiring harness leading to the speed sensor and instrument cluster for any damage, corrosion, or loose connections. A broken wire can interrupt the signal.
* Instrument Cluster (Gauge Cluster): A problem within the instrument cluster itself is possible. This could be a faulty circuit board, a bad speedometer stepper motor (if it's an analog gauge), or simply a loose connection inside the cluster. This is a more complex repair and might require professional help or replacement.
Less Common, More Complex Issues:
* Transmission problems: While less likely, a serious transmission issue could affect the speed sensor's ability to accurately read the output shaft speed.
* PCM (Powertrain Control Module): The PCM processes the signal from the speed sensor. A faulty PCM is less likely, but possible. This is an expensive repair.
Troubleshooting Steps:
1. Check the fuses: This is the quickest and easiest first step.
2. Inspect the wiring: Look for any obvious damage to the wiring harness leading to the speed sensor and the instrument cluster.
3. Check the speed sensor: This usually involves locating the sensor (often near the transmission), disconnecting it, and inspecting it for damage. You may need to use a multimeter to test its output signal (this requires some basic electrical knowledge).
4. Have the vehicle scanned for diagnostic trouble codes (DTCs): A code reader can help pinpoint the exact problem. This is a relatively inexpensive service from most auto parts stores.
If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic for diagnosis and repair. Replacing the speed sensor is a relatively straightforward job for someone with basic mechanical skills, but misdiagnosis can lead to unnecessary parts replacement and wasted money.
