Can you get a service engine code on 91 corsica without diagnostic computer?

No, you can't reliably get a specific service engine code (check engine light code) on a 1991 Buick Corsica without a diagnostic computer or code reader. While some older vehicles had methods to get codes via blinking lights from a diagnostic connector, relying on those methods for a '91 Corsica is unreliable and likely won't work. The system is too sophisticated for a simple blink code interpretation. You need a scan tool.