another code question
 

I've checked codes on my '90 NA but either I read them wrong or my ecu is doing something weird. I connected the wire and turned the key and once it started into the codes it gave me code 21 (which doesn't exist that i can find) and 27, then it gave me 12 and 18 and continuted repeating 12 and 18. I couldn't get the car to repeat the first two codes even if i disconnected the wires. Is this something that could happen? or did I just completely misread it?
Patrick