On Mon, 21 Sep 2015 12:46:48 +0000 (UTC), Ewald Böhm wrote:

But it doesn't answer HOW.

http://www.greencarreports.com/news/...estions/page-2

Based on discussions with knowledgeable sources, we surmise that once an
emissions test was detected, VW got the affected TDI engines to meet the
Tier 2, Bin 5 NOx limits by reducing the fuel flow rate.

This would reduce performance, but most likely not to the point where the
car couldn't complete the emission cycles.

Lowering fuel flow would also reduce combustion temperatures and/or the
duration of high-temperature operation enough to keep NOx emissions barely
within EPA limits.

If the car detected that it was no longer in "testing mode" but had
returned to "driving mode," it would restore fuel flow to the regular
level--which would send NOx emissions soaring.