Overboost condition would result in boost cut and an engine cut to the car when on full boost which would have freaked you out a bit, and a CEL afterwards, which is cleared automatically after a few engine restarts.

Usually ASG up the boost cut though.

If it stays on, the i guess rear 02 sensor maybe. Hard to tell without ODB scanner. Take it back to ASG to get the code read, or get soemone from here to check then go from there.

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Should have bought an STI