Failure in linear equation soluation

Dear Alborz,  In my experience, a variety of problems could result in this error message.   If you set MOP(1)=1, you will get a one-line message every time step that gives you information about how the code is running.  Look at which elements tend to have the maximum residuals - this can be where problems occur.  In some versions of TOUGH you can specify a particular element to watch, with the variable ELST.  If you set up your problem to provide a full printout at a time shortly before it fails (using the TIMES block), and rerun it, you can look at all the elements in your grid for problematic behavior.  You can also try running the problem with a smaller value of RE1, or a smaller maximum allowed time step, and see if that helps.  Good luck, Christine