Q. The "check engine light" lit up on my 1999 Honda Civic. The parts store's code reader indicated that it was the upstream oxygen sensor heater, but also suggested that wire connections be checked before replacement. When I removed the heat shield of the exhaust manifold to access the oxygen sensor, I noted an inch-long crack in the manifold close to the oxygen sensor receptacle Do the manifold and the oxygen sensor need to be replaced or could it be one or the other? The car rarely gets driven (70,000 miles). It has never overheated, so I am also curious why the manifold cracked and whether it is a through-and-through crack. Is a junkyard manifold a good choice for a replacement part? Is replacing the exhaust manifold on this car a DIY job? By the way, the car runs fine, I cannot smell exhaust or audibly detect an exhaust leak.