General Motors Corp."s in-vehicle satellite communications service, OnStar, is now offering subscribers a vehicle diagnostic service that automatically checks on four GM vehicle operating systems and sends reports on how the vehicle is doing to the owner via e-mail.

The OnStar Vehicle Diagnostics system -- which will check the engine and transmission, antilock brakes, airbags and the OnStar system itself -- will send the results of its tests every month, OnStar announced Tuesday. The e-mail report will also provide maintenance reminders based on the vehicle"s odometer reading, remaining engine oil life and other information.

GM is offering OnStar subscribers the new monthly diagnostic service at no additional cost. It can be activated on 2004 or newer GM vehicles, OnStar said.

"OnStar Vehicle Diagnostics gives our subscribers added confidence that their vehicles are ready for the road," said Chet Huber, president of OnStar, in the statement. "By combining OnStar"s vehicle diagnostics e-mail service with our in-vehicle GM Goodwrench Remote Diagnostics service, our subscribers receive relevant and valuable information about their GM vehicle on their computer or directly from the OnStar adviser while driving."

The key benefit that the manufacturer derives from the diagnostic system is the ability to diagnose problems before they become too serious, said Neal Oddes, director of product research and analysis at J.D. Power & Associates in Westlake Village, Calif.

"If they can identify a huge problem before it becomes a recall (situation), that is beneficial," he said.