By combining the C III λλ1906+1909 with the C II λ4267 line intensities it is possible to determine T(C++); this temperature is, in general, considerably smaller than T(O++), the temperature derived from the O III λ4363 to λ5007 intensity ratio. The authors study possible causes for this difference. They show that in the presence of spatial temperature fluctuations T(C++) < T(O++). The authors find that the objects with the highest T(O++)-T(C++) values are those that show large velocities and complex velocity fields, therefore they suggest that the deposition of mechanical energy by the stellar winds of PNe is mainly responsible for the temperature differences. Based on these arguments and the similar T(C++), the Balmer continuum temperature and T(He I) values, obtained for well observed objects, the authors propose that the N(C++)/N(H+) and N(O++)/N(H+) values derived from the ratio of collisionally excited lines to Hβ should be based on T(C++) instead of T(O++); alternatively the abundance ratios derived from recombination line intensity ratios are almost independent of the adopted temperature, and consequently are more reliable.