If a 1995 Toyota Corolla check engine light has come on and the OBD is giving an error code 71 - EGR system how do you diagnose and repair this problem?

My Toyota manual for a 1991 Corolla DX says that error code #71 is used only for California specifications. Here is the official diagnosis, according to the manual: the EGR gas temperature sensor signal (THG) is below the total temperature of the intake air temperature plus 55 deg Cent. (99 degrees F) after driving for 25 seconds in EGR operation range. The manual then lists five trouble areas: 1- Open in EGR temperature sensor circuit; 2- Open in VSV circuit for EGR, 3- EGR vacuum hose disconnected, valve stuck; 4- Clogged in EGR gas passage; 5 - ECU.