I have a 1991 Honda Civic with the check engine light on. I had to downshift on the expressway and had drove about a mile at 65mph in 4th before I shifted to 5th. Not sure if the problem has to do with this, but the light came on shortly after that. The light goes out when the key is turned off and when I drive below 60mph the light doesn't come on, but once I go on the expressway and hit above 6 the light comes on and stays on. Thanks for any help in direction on this.

Begin by checking for codes:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-6.html#post832012

Afterwards, reset the ECM by removing the 7.5A Backup Fuse in the under hood fuse box, or by disconnecting the negative battery cable, for 10 seconds.