There's a tradeoff in these (many? most? all?) diesel engines between better emissions performance and better mileage. The cheat was that the software would tun

There's a tradeoff in these (many? most? all?) diesel engines between better emissions performance and better mileage. The cheat was that the software would tune the engine for mileage normally, except when it detected that it was being tested it would tune the engine for lower emissions. The fix is to lock them into the low-emissions mode.