Thermally stimulated current spectroscopy (TSC) has been applied to characterize deep traps in high-purity semi-insulating 6H-SiC substrates. By using above bandgap to sub-bandgap light for illumination at 83 K and different applied biases, at least nine TSC traps in the temperature range of 80 to 400 K can be consistently observed. It is found that TSC peaks for T < 130 K are significantly affected by light and some peaks are strongly enhanced by the applied bias. Measured trap activation energies range from 0.15 eV to 0.76 eV. Theoretical fittings of selected traps give more accurate trap parameters. Based on literature results connected with deep traps in conductive 6H-SiC, the origin of these TSC traps is discussed.