* Vehicle Speed Sensor (VSS): This is the most likely culprit. The VSS measures the speed of the vehicle and sends this information to both the speedometer and the ECU. A faulty VSS will prevent the speedometer from working and can trigger the service engine light because the ECU needs speed data for various calculations (fuel injection, transmission control, etc.).
* Speedometer Cable (if applicable): Older vehicles might use a mechanical cable to connect the transmission to the speedometer. A broken or damaged cable will cause the speedometer to fail. This won't directly trigger the service engine light, however.
* Powertrain Control Module (PCM) / ECU: The PCM is the computer that controls the engine and other systems. A faulty PCM can affect many aspects of the vehicle's operation, including the speedometer and triggering the service engine light. This is less likely than a VSS issue but possible.
* Wiring Harness Issues: Damaged, corroded, or loose wiring related to the VSS or the PCM can interrupt the signal flow, causing both problems.
* Fuse: Check the fuses related to the instrument cluster and the PCM. A blown fuse could be the simple solution.
* ABS Sensor (less likely): While less directly connected, a faulty ABS (Anti-lock Braking System) sensor *could* sometimes indirectly affect the speedometer reading and potentially trigger a service engine light, particularly if the ABS and speedometer share some wiring or processing.
What to do:
1. Check the fuses: This is the easiest and cheapest thing to check first.
2. Get the Diagnostic Trouble Codes (DTCs): This is crucial. A code reader (available at auto parts stores or online) will read the codes stored in the ECU. These codes will pinpoint the likely source of the problem. The code will often specify a sensor or circuit related to speed.
3. Inspect the Vehicle Speed Sensor: Locate the VSS (its location varies by vehicle make and model – consult your repair manual or an online forum). Visually inspect it for damage or loose connections.
4. Check the Wiring: Examine the wiring harness leading to the VSS and the PCM for any damage, corrosion, or loose connections.
Important Note: Don't attempt extensive repairs unless you have the necessary mechanical skills and knowledge. If you're unsure, take your vehicle to a qualified mechanic for diagnosis and repair. Driving with a malfunctioning speedometer can be unsafe, and ignoring the service engine light could lead to more significant problems.
