The truck will slowly become sluggish. Won't cut out. Shifts nice. Runs fine. Just not much power. Check engine light is on. Did try and get diagnostics the system will not read codes. Like no power to it. Replaced plugs,wires,fuel filter,ignition module, coil, throttlle positioning sensor. Map sensor. Nothing changed. This does occur at random times. Not always this way. 4.3 vortec 185,000 miles.