I double checked on the
http://www.toyota-supra.info/forums/mkiii-supra/9950-tsrm.html , and your correct about the code 12, except it is an 'RPM Signal malfunction'. However, all codes are still two digit. If you believe the problem is the 'Vane Air Flow Circuit', this must be with the air flow meter, which is a vane style meter. Check the connections, and check the meter box itself to make sure the problem is not internal to the sensors in the meter. Next, you'll need to check for continuity in the lines to the ECM.