Implementing SIMPLE algorithm in matlab for solving averaged Navier stokes equation

I'm going to solve navier stokes equation in matlab using the SIMPLE algorithm. The equation is in 2D and derived by averaging in z direction for a parallel plate flow. My code always diverges and end up with very huge velocities and pressures. Since NS equations are averaged in z direction, the height between parallel plates apear in the equation and it is very small. (1e-5 m) If anyone knows some details or available somewhere to read in debugging diverging matlab codes for SIMPLE or CFD applications please let me know.