In reply to:

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It could very well be that five-second pause. Perhaps the engine start is happening right at the instant the tuner is doing some kind of initialization.

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I had a similar issue, in that the engine heater on my car takes about that amount of time, and so I'd be starting the engine at about the same time as the player would be trying to talk to the tuner.

I enabled Hijack's "fake tuner" setting for DC mode, and it's worked flawlessly ever since. (Thanks, Mark! )