FVU is a program developed by NSDL, which is used to ascertain whether the E-TCS return file contains any format level error(s). When you pass an E-TCS return through FVU, it generates an ‘error/response file’. If there are no errors in the E-TCS return file, the error / response file will be display the control totals. If there are errors, the error/response file will display the error location and error code along with the error code description. Incase you find an error, you can rectify it and pass the E-TCS returns file again through the FVU till you get an error-free file.