Which DTC code reader
 

Is there any recommendation for a DTC reader?
I'd like to buy an interface to connect to the PC and have a good freeware.

Something that I am not sure; do the commercially available DTC readers read the codes other than for the power train (Pxxxx)? Right now I would like to check any code on the climate control and that seems to start with a C like chassis.

Thanks
H.