[christian]

I'm not sure we know yet. CARB/EPA isn't saying that that was affected - just the certification test.

But the dumbest (simplest) programming for this would be to use the ABS wheel sensors to tell anytime the car is on a dyno (which would encompass almost all testing):

Get Wheelspeed(Rear-Left) as WRL (Cars have wheelspeed sensors on all four wheels to allow the usage of ABS and traction control systems)
Get Wheelspeed(Rear-Right) as WRR
Get Wheelspeed(Front-Left) as WFL
Get Wheelspeed(Front-Right) as WFR
Get RPM as RPM
WheelspeedRear = avg(WRL, WRR)
WheelspeedFront = avg(WFL, WFR)
If WheelspeedRear < WheelspeedFront
(the car is on a dyno)
And RPM > 0
(the car is running)
Do NOxCatalyzer

That's terrible dummy code, but you get the idea. It's really, really simple. Obviously, the VW code would be more sophisticated, but this would be one way to do it.