Recently the speedometer failed but the instrument cluster lights up OK.
Additionally the check engine light has come on.

How to proceed diagnosising both issues?

Thank you for any help!

Ironically I just replaced one in my wife's '93 civic on Sunday. Same issues. The speedometer fluttered and fluctuated for a couple of weeks (if not longer) before quitting all together. Diagnosis is simple but in the same note aggravating. The engine light is an indicator that your sensor is bad. First you have to know what makes the speedometer operate. This is the VSS - Vehicle Speed Sensor which is located on the transaxle (passenger side). You can reach the VSS through the engine compartment. There are a few hoses and wires you have to work around but nonetheless access is fairly easy. Check for loose or this - connected wires or even corrosion at the VSS. Check for voltage at the connector to the VSS. You should have voltage. If there is no voltage/battery then check for open circuits bewtween the VSS and the fuse box. Check the black wire of the connector for continuity to ground. Raise and place the front of your car onto jackstands (securely w/ back wheels blocked). Shift your car into neutral. Make sure the VSS is connected and turn the ignition to ON position (don't start the car) the backprobe the yellow or white wire at the VSS connector w/ the positive lead (voltmeter) connect the negative lead (voltmeter) to the body ground. This next part takes two people unless your talented. While holding one wheel steady, turn the other wheel by hand. While you are doing this there should be a pulse on the voltmeter between 0 to 5 volts. If there is nothing your sensor is defective/bad. If this is the case be prepared to shell out at least $100 for a new VSS (autozone). There are two bolts that hold the VSS. This a ten minute job tops. Good luck.

Read sections A and B below for diagnosing the CEL coming on:

https://www.askmehelpdesk.com/cars-t...das-46563.html