It's been just over three months since the U.S. Environmental Protection Agency (EPA) first revealed that Volkswagen diesel cars were equipped with "defeat device" software that allowed them to cheat emissions tests.

But U.S. owners of the 482,000 2009 through 2015 models equipped with EA189 2.0-liter four-cylinder engines will have to continue waiting for modifications a little bit longer.

Last month, Volkswagen submitted a proposed update for these cars to the EPA and the California Air Resources Board (CARB).

DON'T MISS: Volkswagen Proposes 2.0-Liter Diesel Fixes As Crisis Continues

And now the California agency will extend its deadline to approve or reject that proposal, according to Reuters.

In a letter to VW, CARB said that after getting updates from the carmaker on its proposed plan, it will extend its deadline to review that plan by about three weeks.

No details of the proposed changes have been been made public since Volkswagen submitted its proposal to regulatory agencies in late November.

2014 Volkswagen Passat TDI

2014 Volkswagen Passat TDI

That leaves owners in a holding pattern while the EPA and CARB review the plan. Either or both of the two agencies may request revisions that VW will have to address, further prolonging the process.

Volkswagen also hasn't discussed a timeline for expected approval of the fix, or the beginning of recall repairs on the affected TDI models.

Under the best possible circumstances, recalls that involve only software updates might begin sometime early next year and continue through spring and summer.

ALSO SEE: How Will VW Fix My Diesel Car, And When? A List Of All Models

But some vehicles may need to have a Selective Catalytic Reduction system fitted, which comprises a new catalytic converter, a tank for urea exhaust-aftertreatment fluid, and all of the associated hardware to inject it into the exhaust.

This could cost several thousand dollars per car, and may push recall repairs into 2017. Industry analysts have expressed skepticism that such radical changes are justified for cars as much as seven years old, and suggested that buybacks may be the cheaper route.

VW notes that all affected cars remain safe to drive, and that owners continue to do so.

2014 Volkswagen Beetle TDI

2014 Volkswagen Beetle TDI

Meanwhile, U.S. regulators are also waiting for a proposed fix for the estimated 85,000 VW Group diesels sold in the U.S. with a separate, 3.0-liter V-6 powertrain.

Audi CEO Rupert Stadler has said the carmaker believes problems with these vehicles can be addressed with software fixes alone.

MORE: Audi V-6 TDI Diesels May Require Only Simple Fixes: CEO

Volkswagen does plan to begin recalling some of 11 million affected diesels in Europe next month for updates.

Modifications to software (and in one case an inlet tube) have been approved by regulatory authorities there, who oversee emissions standards far less stringent than those in effect in the U.S. since 2008.


Follow GreenCarReports on Facebook and Twitter