General Motors Corp.’s OnStar telematics service says it has a bona fide hit on its hands with its OnStar Vehicle Diagnostics monthly e-mail program.

OnStar says subscriptions to the service, which is available free to all eligible OnStar subscribers, already has topped the million mark and is climbing rapidly.

“Everything in our business is scaling like crazy,” says OnStar President Chet Huber.

By the end of the year, Huber tells Ward's, that number should double to more than 2 million.

The OnStar Vehicle Diagnostics service monitors vital information retrieved from the subscriber’s car and automatically sends monthly e-mails to owners alerting them of potential problems or when it is time for routine service actions, such as oil changes and tire rotations.

The program was launched a little over six months ago, and although customers initially were wary of the service – doubtful it would continue to be offered free of charge, many now consider it a key OnStar feature.

“They figured after 90 days we would try and charge them for it,” Huber says.

GM dealers also were skeptical at first, Huber says, but they now see the e-mail service as a key brand differentiator and a unique way for them to maintain contact with customers over the life of the vehicle ownership, because the e-mails provide a contact point for the owner with the dealer where the vehicle was purchased.

OnStar recently added new features to the service. It now will alert vehicle owners of recalls and other service actions and provide information on their XM Radio subscriptions.

The e-mail also indicates whether the vehicle can run on E85. Once development is completed over the next several months, recipients will be able to click a link in the e-mail and find an up-to-date list of fuel stations in the U.S. that carry E85. Huber says drivers also will be able to contact OnStar while on the road and get information on the nearest E85 station for a service adviser.

In June, diagnostic information will be expanded. Currently, diagnostic results are provided for several broad categories of systems, such as Engine/Transmission, Airbag and Antilock Brakes. But OnStar will begin to show detailed information on subsystems within those categories.

The e-mail service now is available in Spanish.

OnStar currently has more than 4 million subscribers. Huber won’t predict how many ultimately will subscribe to the e-mail service, but he says “we think we’ll get the overwhelming majority of people that have e-mail addresses to sign up for the service. The issue now is awareness and understanding of what it is.”

That is improving, Huber says, thanks in part to a series of TV commercials focused on the e-mail service and better informed dealers that are using OnStar’s features as sales tools.

“There’s a growing awareness, and people are coming in to dealers and asking about (the service) now,” he says.