A properly functioning speedometer in a Honda Civic should rest when the car is idling and should slowly rise in response to the acceleration of the car. When a speedometer malfunctions, it may function in many different ways, including constant idling, rising when the car is idle, reporting an incorrect speed, and bobbing up and down. When a Civic's speedometer malfunctions, it could be caused by a number of problems.
The most common cause of speedometer malfunction is loose wires that plug into the back of the speedometer. Loose wires are often caused by improper placement when the vehicle was manufactured, or due to driving repeatedly over rough terrain.
A short circuit will cause the speedometer in a Honda Civic to malfunction. Oftentimes, a short circuit is the result of the plastic clip breaking that holds the wires away from the engine. Without a clip, the wires will rub freely against the engine, eventually resulting in bare wires that short out against the metal of the car.
Variable Speed Sensor
The Variable Speed Sensor, also known as the VSS, is located on the transmission and will cause the speedometer to remain idle during acceleration if damaged. Corrosion on the connection could also result in speedometer malfunction, and is generally corrected by cleaning, repair or replacement.
Exhaust Gas Recirculation System
The Exhaust Gas Recirculation System, often referred to as the EGR, is a valve on the engine that opens to allow exhaust to pass through. If the EGR goes bad or becomes damaged, it may remain open when it is supposed to be closed, which will cause the speedometer to bob up and down sporadically, and will also cause the engine to run very roughly.
Every Honda Civic has a fuse box, which is located on the inside of the dashboard on the driver's side. If the fuse that controls the speedometer comes loose or blows, the speedometer may behave erratically, or stop functioning entirely. A blown fuse may be identified by a black blemish in the plastic.
Unlike most other cars, the Honda Civic has two oxygen sensors, which relay exhaust information to the car's computer so that the fuel supply and ignition rate of the engine remain stable and efficient. If one or both oxygen sensors malfunction, incorrect ignition and fuel supply could cause the engine to run poorly, which will cause the speedometer to jerk and stutter in response. A bad oxygen sensor is often accompanied by the check engine light.