The net result is that real-world emissions are 30 or more times higher than those in the tests.

Lange also noted that the automotive industry is particularly careful to document its entire development process. All software changes are tied to requirements documents and those documents are signed-off by multiple layers of management in the companies. So, unless there is a cover up, VW should be able to determine exactly who made the change and when it was done. It is "completely impossible" that some kind of rogue engineer simply inserted the cheating code into ECU, he said.

The engine ECU is typically provided by an outside company (Bosch in the VW cars) and runs proprietary code that contains a computer model of the engine. Car makers cannot change (or even see) that code, but the model is driven by some 20,000 variables that describe the engine and its functioning. There is just as much record-keeping in how and why those variables get changed as there is with other changes, so VW (or the investigators) should be able to see just how this all came about.

[...]

It turns out that the standard SCR model does not work under all conditions (e.g. if the engine is too hot), so there is an alternative model that runs in parallel. It is a much simpler model, with fewer inputs, that has the goal of never adding too much AdBlue. There is code in the ECU that determines which model to use, and that code depends on the data provided by the car maker. In addition, the ECU stores information about which model is chosen at each ten-millisecond interval.

The conditions that determine which model is chosen are all ORed together to decide when to switch to the alternate model. Many of those conditions were impossible (e.g. air temperature greater than 3276.8°K or less than 0.1°K), but one was particularly strange since it always evaluated to true (engine temperature greater than -3276.8°K), which meant that the OR would evaluate to true, thus the alternative model should always be chosen.