The crank position sensor is mounted at the top/driver's side of the transmission bellhouse and is only used for monitoring by the ECM, meaning it will not create an engine performance or drivability issue. It can be caused by a chipped or broken flexplate or flywheel ring gear, a bad crank position sensor or due to an open, short or high resistance in the harness or connectors. I don't see where flushing your coolant would have caused this. First thing I would do is clean-up the two, EGI ground points by removing the 10MM head bolts and cleaning up the eyelets and intake plenum contact area with a small wire brush, Scotch pad, steel wool or light sandpaper. Re-assemble. Check your battery cable connections to make sure they are clean and tight. Erase the stored codes and see if the code returns. If it doesn't, problem solved. If it does, then you'll have to do a little more diagnostic work or take a gamble that it's the sensor, itself, and replace it (it's a bit of a pain to get to, especially if you have four wheel drive). Diagnostic information for that code is listed in the Engine Control chapter of the factory service manual, which you can find in the "knowledge base" section @ NissanHelp.com.