Synthetic UV spectra (black) from consistent models for NGC 2392, compared to the observed UV spectra (red and blue). The top panel show the spectrum from a model using the stellar parameters from the analysis of K06. The spectrum is clearly incompatible with the observations, thus ruling out this parameter set. The center panel shows the spectrum from a model using the parameters from the analysis of P04. The spectrum predicted by the model is generally in good agreement with the observations, but the O vi line is not reproduced. In the bottom panel, shocks in the wind have additionally been included in the modeling procedure. The small amount of high-energy radiation (L_X/L_bol ~ 10^-6) generated by these shocks is enough to ionize a small fraction of oxygen into O vi, sufficient to reproduce the observed line strength. This result indicates strongly that shocks and the X-ray radiation emitted by them play a role not only in the winds of massive O stars, but also in the winds of O-type CSPNs.