Open the best solution manual you have. Compare your discretization coefficients with theirs. Did you forget to linearize the source term ( S_c ) and ( S_p )? Are your boundary conditions implemented correctly? The manual acts as a debugger.
Because Patankar’s book expects you to write code (typically in Fortran or C), the best solution manuals include for the iterative solvers. For the iconic Problem 5.2 (Lid-driven cavity flow), the manual should explain: Open the best solution manual you have