While Volkswagen has submitted details of proposed fixes for its 2.0-liter four-cylinder TDI diesel models to the EPA, no details of the changes required have emerged.
But for the other set of VW Group diesel engines, the 3.0-liter V-6 TDI units used in Audi, Porsche, and Volkswagen vehicles, Audi's CEO says the fix may be simple.
That V-6 is used in seven different 2009 to 2016 vehicles, with total U.S. sales of 85,000 cars across all models.
Rupert Stadler told a group of Audi workers at the company's Ingolstadt headquarters that necessary modifications are moving toward a decision.
"Swift, straightforward and customer-friendly solutions are in discussion," Stadler said to the gathering, according to Reuters.
"Every day we are taking another step toward the solution."
2014 Audi Q5 TDI, Catskill Mountains, Oct 2013
It remains unclear whether the company's U.S. arm simply needs to "revise, document in detail, and resubmit for U.S. approval" the engines' emission control software--as it said in November--or whether modifications will be required.
To fix several million of its four-cylinder diesel engines to meet European emission rules--which are much more lenient than U.S. rules--the company only needed to update the software on three different sizes of engine, and fit a newly-designed air intake tube to one version.
The U.S. Environmental Protection Agency widened its investigations to VW Group's 3.0-liter V-6 TDI engines in September, several days after it disclosed Volkswagen's admission that it had installed "defeat device" software on its 2.0-liter four-cylinder TDI engines.
The V-6 powerplant is used in a variety of higher-priced vehicles from the VW Group, the bulk of them Audi sedan and SUV models.
2013 Porsche Cayenne Diesel
The 85,000 vehicles affected are five Audi models: The A6, A8, and A8L TDI sedans; the A7 TDI hatchback; and the Q5 TDI sport utility vehicle, along with the Volkswagen Touareg TDI and the Porsche Cayenne Diesel.
While VW's German headquarters had initially denied the EPA's charges on the V-6 diesel, its U.S. operations for each of the three brands pulled both new and used models of the affected cars off sale at its dealerships.
But much remains to be learned about the details of the fixes on both sets of engines, though the degree of transparency that VW Group will provide in its internal investigations of the cheating scandal remains under debate.
European business analysts have slammed VW's handling of the crisis, now entering its fourth month, for its opacity, sometimes combative tone, and lack of communication with owners.
It took almost eight weeks for VW Group of America to offer its 2.0-liter TDI owners $1,000 in gift cards--$500 of which could only be redeemed at a VW dealer--as well as expanded roadside assistance.
The company continues to stress that all the affected diesel models are safe and can continue to be driven until modifications are identified and approved by regulators.