I just did a search, and the Interfire A-800.5 is an inexpensive aftermarket 5 channel amplifier. Is there any chance that your mechanic removed your OE DSP amplifier and installed an aftermarket amplifier in its place? If so, he could have done some severe damage to your car's wiring.

If he exchanged the DSP amp for an aftermarket amp, that would explain why DSP is not detected and why it sounds flat. All of the DSP processing is in the DSP amp, and it is a required component for the car. By the way, an aftermarket amp does not need coding at all.

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