In May 2009, during the fourth space walk of the HST Servicing Mission 4 (SM4),
the STIS Side-2 circuit board containing the failed low voltage power supply was replaced. This repair fully restored the STIS instrument to operation with capabilities similar to those it had before the Side-2 failure. Although the performance of STIS post SM4 is similar to its performance prior to the 2004 failure, there were some important differences. These include increases in the CCD readnoise, the NUV-MAMA dark current, the CCD dark current, the CCD Charge Transfer Inefficiency and modestly decreased sensitivity of all channels. More details on these changes can be found in the STIS Instrument Handbook.