I have a 91 na coupe and i cant seem to clear the ecu. i've disconnected the -battery cable for like 30mins and it didnt clear still giving me the same error code (10).

I'm totally talking out of my butt here, since I don't know what a 10 error is, but if the condition which sets the code is not fixed, the code will reappear immediately.

Code 10 is the intake air thermosensor located on the AFM. This sensor doesn't usually go bad. The problem may be nothing more than the sensor plug has come loose or is not plugged in tightly. Remove the plug and examine the pins to make sure none of them are bent. Plug in in and clear again. If code 10 still shows, you may very well indeed have to replace the sensor or the AFM.

Iirc, don't you need to press the brake pedal after the battery is disconnected, to clear the ecu?

if the code comes right back, maybe the part for that code is bad....

I haven't had to do this.

I know you need to do this on the S4's, unless I'm getting it confused with the Z that's in the shop now.

Ah, well I've never had to clear codes on my old S4.