On Tuesday (US time), Mary Nichols, chair of the CARB, told Volkswagen in a formal rejection letter that the company's submission had been declined as "the plans contain gaps and lack sufficient detail", that the "descriptions of proposed repairs lack enough information for a technical evaluation of their feasibility and likelihood of success", and "the proposals do not adequately address overall impacts on vehicle performance and durability".
She also noted that Volkswagen failed to meet state regulations that its "emission recall plans must correct the problems in an expeditious manner".
The CARB's rejection only applies to Volkswagen and Audi vehicles fitted with the company's 2.0-liter EA189 turbo-diesel engines. Installed in vehicles from the 2009 model year onwards, the cars employed "defeat devices" that can detect standardised laboratory emissions testing and generate compliant results.
Although the CARB's rejection only involves an estimated 75,688 EA189-equipped cars registered in the state, the EPA swiftly followed suit and publicly rejected Volkswagen's proposed fixes. In a statement the authority said: "[The] EPA agrees with CARB that Volkswagen has not submitted an approvable recall plan to bring the vehicles into compliance and reduce pollution. EPA has conveyed this to the company previously."
It's estimated that nearly half-a-million affected vehicles have been sold in the US with the EA189 diesel motor.
In November 2015, American authorities also cited the Volkswagen Group's 3.0-litre V6 TDI engine for emissions testing violations. The CARB has yet to receive a proposed solution from Volkswagen as the deadline for submission is February 2, 2016.
The non-compliant Audi-developed V6 turbo-diesel engine is estimated to have featured in around 85,000 Volkswagen, Audi and Porsche models sold in the US since 2008.