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[jserrano]

I'm not entirely familiar with MSD ignition but I can tell you about code 21 from the OEM perspective.

The ECU uses a condenser/resistor component to pick up a sampling signal from the primary side of the ignition coil. This feedback signal is called the Ignition Check. You can think of it like an engine "heartbeat" check. When the ECU misses this signal during engine cranking or running, it believes that there is a fault within the primary side of the ignition. If you have access to an oscilloscope you could test pin 3 of the ECU to see if the signal is missing or has the wrong waveshape or amplitude.

Unless you've changed something in the ignition side, I'm more inclined to think that something is wrong with the MSD ignition. Asleep, and a few others, have a good deal of experience with MSD. Perhaps they can throw in a few more words on this...