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2003 sierra 2500HD 6.0

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getting reduced engine power on and off for a year. checked and replaced engine grounds ran good for 2 months. now back to reduced power DTC PO 1516 and 1518.I've changed Throttle body , TB connector, TAC module, APP sensor, ignition switch,  PCM. I should explain that this truck was laid up for some time prior to me buying, it  had 133k on it originally It was a water co. service truck. I'm at a loss for why I keeps throwing the codes. I've recently changed MAF senso ras a last ditch effort to remedy the problem. I should mention that I can city drive for weeks with no problem but the minute I hit highway and speed up to 60 check engine light flashes and codes say multiple misfires and knock sensor fault. I can't feel any of this. I suspect crank relearn is needed but I don't know if this is related to reduced engine power. I could use some feed back

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