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Subsection 8.1.3: Boundary Conditions Up Chapter 8: Troubleshooting Section 8.3: Common Issues

## 8.2 Debugging a Model

When a model is not running as expected, the first thing to try is rerun the problem using FEBio's debug mode. Debug mode can be activated by adding a –g on the command line. For example,
>febio –i file.feb -g

When in debug mode, FEBio will do additional checks during the solution (e.g. check for NAN's, check for zeroes on the diagonal of the stiffness matrix, etc.) and will store all non-converged states to the plot file. Inspecting these non-converged states can often be useful to identify the cause of the problem. (E.g. when the model seems to disappear at some point might indicate a rigid body mode.)
Since storing all non-converged states may make the plot-file too large for post-processing, debug mode can also be turned on during the analysis. To do this, start the problem normally (without the debug flag –g). Then, right before the problem starts to fail, interrupt the run (using ctrl+c) and wait for the FEBio prompt to appear. Once it appears, enter debug [ENTER], and then cont [ENTER] to continue the run in debug mode.
Subsection 8.1.3: Boundary Conditions Up Chapter 8: Troubleshooting Section 8.3: Common Issues