I just replaced quite a few parts on my 91 civic. Battery, Alternator, Drivers Side axle, radiator hoses, thermostat. After installing the new battery, I attempted to start the car. When turning the ignition on, the check engine light repeatedly turns on and off. The starter turns the engine over fine when I advance the key to actually try to start the car. The only thing that I have not completed is filling the gearbox with gear oil.

Perform steps in Sections A and B below:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563.html

Clean the main ECM ground on the thermostat and see if that solves the problem. If not, be sure to run the K-Test, since the ECM could be experiencing intermittent failure.

As you as you mentioned clean the ground on the thermostat, I remembered I forgot to reinstall the ground. The car fired right up afterwards. Thanks, this was a huge help. Very fast response also!!