Here is a page from shbox.com. http://shbox.com/1/Dtcs.htm
. The '93 ECM tested solenoid or relay circuits in groups. They call the error a QUAD driver XX. The 26 is a quad driver module (QDM #1) open/shorted air pump relay or EGR solinoid or EVAP solinoid. You have to figure out which of the 3 are defective. The DTC 28 is QDM #3 is open/shorter AC relay or fan relays. If you have any of the circuits open (deleted) or the fuses are blown you will see those DTC's. The DTC 21, as coco mentioned, is a Throttle Position sensor high, it says you TPS voltage to the ECM is reading to high at idle (over 2.4 volts) -or- to high when running (over 4.5 volts). Reset it, if it comes back could be bad TPS or a bad connection/wire from TPS to ECM.
I'm Bob.. 1994 Z28, A4, Stall, CAI, Cam, RR's, 342 rear, LT's , CatBack, Tuned
Last edited by bobdec; 11-20-2012 at 10:28 AM.