2003 Hyundai Elantra GT:Take out 4 Screws, Engine heat shield over Oxygen Sensor

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2003 Hyundai Elantra GT: I take out 4 rusted/corroded screws/Bolts for the Heat Shield and change the Oxygen Sensor, upstream, top one, and there is a metal heat shield over it and attached to the manifold. **The 4 bolts/screws are rusted/corroded in and haven't probably needed to be removed since year 2002 when the car was made. I was told by a longtime/experienced mechanic NOT to use an "Air compressor gun" I tried BP Blaster and taking a LONG time!! **How can I get them out??