Since you already checked GNttype.org, I would suggest getting a copy of the code 42 flowchart and using a DVOM (less than 10megaohm impedance) follow the flowchart to determine what needs to be replaced. Here are some notes:
1) Check to see if the ECM recognizes the problem (you've done that)
2)With the ECM disconnected, ohm meter reading at ECM termB4 should be less than 200ohm (assuming stock ecm). Higher resistence could mean fault at circuit 423, poor ignition module connection or faulty ign module
3)If test light is on when connected from 12v to ECM harness term D5, either circuit 423 is shorted ground or the ignition module is faulty
4)Check to see if the ignition module switches, when the bypass circuit (B5) is energized by 12v (see 3), through the test light. Resistance between B4 and ground should exceed 8000 ohm. If over 8000 ohm, reconnect ECM and idle for 2 minutes and see if code 42 returns, if so, faulty ECM or if no code 42, intermittent prob
5)Cont on 4, disconnecting the ign module should make the ohm meter read as if it were monitoring an open curcuit (infinite reading). Otherwise circuit 423 is shorted to ground.
PLEASE BE SURE TO USE A OHM METER WITH LESS THAN 10 MEGA OHM IMPEDANCE WHEN PROBING INTO THE ECM, OR CALL RED FOR A REPLACEMENT!!!!!
