I am not very familiar with these validation tests and am looking to just use them in a larger model.
I have an ARMA(P,Q) model and $P=Q=2$ gave me the lowest BIC value and hence I stuck to it. Now, I know people do something with the Ljung-Box Q-test test for autocorrelations. I did this on Matlab with lags of 5,10, 15 and hence degrees of freedom of 1,6,11. More explanation of this Matlab function is here:
[http://www.mathworks.com/help/econ/lbqtest.html#bt080ir-1]

I get $h= [0 0 0]$ and $p=[0.1511,  0.8545, 0.3046]$. Please give me a reason for :

i) if this means my ARMA model is good enough, why so?
ii) if this means my ARMA model is not good enough, then why so and what should I do instead?


  [1]: http://www.mathworks.com/help/econ/lbqtest.html#bt080ir-1