Head of VW has a bumpy, awkward US tour at the auto show in Detroit, with California emissions’ experts and in Washington, DC.

Matthias Mueller proposes a fix to bring the German automaker’s hundreds of thousands of tainted diesel models into compliance with US pollution standards.

The company’s proposed solution in the EU was approved by authorities weeks ago. That fix includes software changes and the installation of a simple plastic tube and mesh device meant to better aim air toward emissions sensors.

“It was a much simpler solution to the one that they will be able to use in the US, because the emission limits for diesel (nitrogen oxides) emissions in Europe are about four times higher than the US limits are,” said Greg Archer, head of the clean vehicles program at the Brussels-based non-profit Transport and Environment.

To meet US clean air standards, VW’s options include selective catalytic reduction, in which the automaker would inject a nitrogen-oxide-trapping chemical called urea into the exhaust pipe to reduce emissions. Archer said VW would be the first to retrofit that type of system in a diesel car, and it would undoubtedly be expensive.

Another option, which VW is reportedly considering, is installing a catalytic converter to trap the nitrogen oxides. Archer said that would be an easier fix but less effective at reducing emissions.

A practical challenge to either option is finding the room within the vehicle to install either a selective catalytic reduction device or a catalytic converter.

“There isn’t a great deal of space on a lot of vehicles, so physically it’s quite difficult to do,” Archer said.

US is not persuaded.   VW’s sales down.