Probably the SMOG test. VW got caught a while back and I’ll bet that is what it is. Puts the engine into a really efficient mode to make it pass the smog test, but move the steering wheel, like when you are actually driving, and it goes back to normal operation, which puts out like 15 times the smog. VW tried to say it was some rogue operator, but it was bullshit and they got caught. So all this crap about being more fuel efficient is a crap-load of lies.
The fuel efficiency isn't the issue, it's the amount of particulates it generates. Diesel is generally more efficient than gasoline, but it also doesn't burn as cleanly. If the left weren't so down on diesel in general, you'd be seeing diesel-electric cars by now, because if you want to do a 'hybrid', that is proven tech.