My code won't stop looping

I am running my code and it gives me the answer for the first 7 of 8 ndiv values but won't for the last one. It will get right to the end of the iterations (somewhere just past 6e5) but then restart looping for the nidx=8 so when I pause it I can see the value is somewhere in the 2e5 and then next time it is 252. How do I make this problem stop so it will give me the final all_k value?

Code:

a=25;

b=a;

ndiv=[4,8,16,32,64,128,256,512];

for nidx=1:length(ndiv)

nx=ndiv(nidx);

nz=nx;

x = linspace(0, a, nx);

z = linspace(0, b, nz);

[X, Z] = meshgrid(x,z);

Tnp1 = zeros(nx, nz);

Tnp1(:,1) = 20; % Left boundary

Tnp1(:,end) = 20; % Right boundary

Tnp1(1,:) = 20+380*sin((x*pi)/25)+205*sin((x*5*pi)/25); % Bottom boundary

Tnp1(end,:) = 20; % Top boundary

% Initialise error and set tolerance for convergence

err = 1;

tol = 1e-8;

k=0;

while err > tol

Tn = Tnp1;

k=k+1;

for i = 2:nx-1

for j = 2:nz-1

Tnp1(i,j) = (1/4)*(Tn(i+1,j)+Tn(i-1,j)+Tn(i,j+1)+Tn(i,j-1));

end

end

err = max(abs(Tnp1(:) - Tn(:)));

end

T2 = Tnp1;

k2=k;

all_k(nidx)=k;

end

Answer 1

[Sulaymon Eshkabilov]

Here is the correted part of the code.

...
while  err > tol
    Told = Tnp1;
    k=k+1;
    for i = 2:nx-1
        for j = 2:nz-1
            Tnp1(i,j) = (1/4)*(Tnp1(i+1,j)+Tnp1(i-1,j)+Tnp1(i,j+1)+Tnp1(i,j-1));
        end
    end
    err = max(max(abs((Tnp1 - Told)./Tnp1)))*100; % Probably error in "%" would be more appropriate here. Otherwise, 100 can be removed 
end
T2 = Tnp1;
k2=k;
all_k(nidx)=k;
end