FRANKFURT, July 31 (Reuters) - Germany's Federal Office for Motor Vehicles has approved diesel engines of General Motors' Opel unit, after initial doubts about whether the engine control software was permissible, weekly Bild am Sonntag reported.

Engines of the 2.0 CDTI type which are built into Insignia, Cascada or Zafira branded cars have been cleared, the paper reported.

Opel had no comment while the Federal Office for Motor Vehicles was not immediately available for comment.

A German investigating committee earlier this year shortlisted 30 car models which showed suspiciously high levels of carbon dioxide (CO2) emissions for further testing after Volkswagen admitted it cheated U.S. emissions tests by installing software capable of deceiving regulators.

Among the carmakers under scrutiny was Opel which had admitted that its Zafira model includes engine software that switches off exhaust treatment systems under certain circumstances.

(Reporting by Arno Schuetze; Editing by Adrian Croft)