When I bought my car a couple of months ago the speedometer/odometer didn't work... the same day I bought it it started working... at first it jumped up and down and then stuck with the speed and worked... a couple of days ago it started jumping around again and now doesn't work at all it's been almost 5 days now and it's jumped around 2 or 3 times since but drops back to zero and stays... it's a 2000 civic si... are there any wires I need to check or what should I do?

. Clean the main ECM ground, likely located on the thermostat housing.

. Check all under dash fuses with a test light or multimeter:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post252145

. Check connector to the Variable Speed Sensor (VSS), located on top of the transmission housing. Remove VSS, clean, and spray with WD-40.

. Replace the VSS, if the problem persists.