How do I check the diagnostic codes on a 1990 Honda Civic? Thanks!

One of the first things you want to do when you have problems with your Civic is to check your ECU / ECM (Electronic Control Unit / Electronic Control Module) for error codes. It is a metallic box, placed under the carpet, below the glove box, where a passenger would usually keep his/her feet while driving. Pull the carpet down a bit, and you'll see it. There will be a circle on the metallic sheet, which is just an opening through which a red LED light can blink. When you turn the key to position "II" (when the Check Engine Light, Oil light and Battery light turn on, but you do NOT yet crank the car) the LED will blink once, as if it's saying "Hello there, I'm alive...". That LED will report any error codes it might have stored after the initial hello-blink. Error codes are coded with blinks as follows: number of short blinks = error code, pause = another error code is coming now. For example, if you're getting a "HELLO_BLINK-pause-blink-pause-blink-blink-blink-blink" it means the ECU stored error codes 1 and 4. Once all error codes are "blinked" (displayed), ECU will "reblink" them again. Notice that if you're getting something like this: "HELLO_BLINK-pause-blink-pause-blink..." it means that ECU has an error code 1 stored. If there are no error codes, then the ECU will just blink once and will not blink again.

After you've written down all your error codes, you can search online for an error code list in the "Sticky."