What does this code P1258 means on a 2001 oldsmobile Aurora 4.0?

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When the system is on or off, push the ON/OFF button down for three seconds to change the display between English and metric. This will also change the digital screen for the climate control system between English and metric.

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pop the trunk and you should see two screws along the rear taillight assembly. remove the screws and the taillight assembly should pop out. you can then twist the bulb and pop it out along with the harness, put the new bulb in and reverse.

If it's your 3rd brake light, the bolts holding it in against the rear window will be found inside your trunk along the top side of your trunk (get in and look up)

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Call autozone and ask them to define the codes for ya then get with your mechanic and get em fixed.

SOURCE: 2001 Oldsmobile Aurora Rear Rattle

If it's making a clunk noise when hitting bumps, check the mounting plates and bushings that hold the rear end to the frame. GM made these plates out of aluminum and the bolts that go through the plate and the bushings rot out and the rear end slams against the frame.

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Autozone repair guide site:
http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?chapterTitle=Water+Pump&partName=Engine+Cooling&pageId=0996b43f8036eab3&partId=0996b43f8036ea84