The 0171, 0174, and 0300 usually mean there's a vacuum leak somewhere. Thousands of words posted here and in Northstar Performance over the years on just these 3 codes. The search tab up in the blue bar will help.
Check the intake piping from the MAF to the throttlebody for leaks, the vacuum connectors at the top- and sides of the throttlebody casting, the intake manifold to throttlebody rubber connector plenum, the PCV piping from the rear cam cover to the front center recess in the intake manifold, and the intake manifold overpressure valve (inside the deep recess under and behind the power steering pump).
Check HOW ????? Go to Walgreen and buy a cheap stethoscope. remove the chest diaphragm and substitute a 2-foot length of vacuum tubing (AutoZone, PepBoys, Advantage, etc.) and snoop for hissy leaks. In the intake manifold end and under the plenum, a shot of choke cleaner or starting fluid will change the idle speed if there is a leak.
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The pic is the intake manifold connector plenum. They have a habit of cracking at the bottom of the flex bellows. Not a hard replacement, but you need some tools, gaskets, and torque specs.
Welcome to the Wonderful World of Cadillac. (Now that you have the car, update your profile, up in User CP, with the year, model, and mileage. It helps to know what you need advice on.)