* Vehicle Speed Sensor (VSS): This is the most common cause. The VSS measures the vehicle's speed and sends that information to the instrument cluster (the gauges). If it fails, both the speedometer and the tachometer (which often relies on speed information for accurate readings at lower RPMs) will stop working. The VSS is usually located on the transmission.
* Instrument Cluster (Gauge Cluster): A faulty instrument cluster itself is another possibility. A short circuit, blown fuse (specifically one related to the instrument panel), or internal failure within the cluster could prevent the gauges from receiving or displaying the signals.
* Speedometer Cable (if equipped): While less likely on a '97 LeSabre (which likely uses electronic signals), some older vehicles used a mechanical cable. A broken or damaged cable would prevent the speedometer from functioning, but it wouldn't directly affect the tachometer. This is less probable given the simultaneous failure.
* Power Issues: A blown fuse, faulty ignition switch, or a problem with the car's power distribution system could cut power to the instrument cluster, disabling both gauges. Check your owner's manual for the relevant fuses.
* PCM (Powertrain Control Module): While less likely, a failing PCM can interrupt signals to the instrument cluster, potentially affecting both the speedometer and tachometer. This is a more involved diagnosis.
Troubleshooting Steps:
1. Check the fuses: This is the easiest first step. Consult your owner's manual to locate the fuses related to the instrument panel and gauges.
2. Inspect the Vehicle Speed Sensor: This often requires getting under the car. Look for the VSS (usually near the transmission) and check for any obvious damage or loose connections. You might need a multimeter to test its output voltage.
3. Have the instrument cluster tested: This may require taking it to an auto electrician or a repair shop specializing in instrument clusters.
4. Check for OBD-II codes: Use an OBD-II scanner to read any diagnostic trouble codes stored by the car's computer. These codes might provide clues to the root cause.
If you're not comfortable working on your car's electrical system, taking it to a mechanic is the safest and most efficient way to diagnose and fix the problem. Describing the simultaneous failure of the speedometer and tachometer will help them pinpoint the likely cause more quickly.
