While driving my 2002 Montero into Ridgecrest (CA) today my air conditioning decided to stop blowing cold air.

It started out blowing a bit of cool air, but didn't get to the desired setting.

Does anyone have some tips on where to start with a diagnosis?
I read through the shop manuel (on-line version), it says I need a "scan tool" (MUT-II) to complete the diagnosis.
Is there a way around using this to figure out what needs fixing?

Thanks in advance.

'02 Montero: OME suspension, ARB front bumper, homemade skid plates & spare tire Blitz can mount, extended diff. breather, K&N air filter, Optima Yellow Top, BajaRack, IPF lights, Goodyear Duratrac 265/75/16's, RUD 4x4 Grip chains