The engineers who designed the engine - yes.
The engineers who coded the software for the engine computer? Why would they know what this does? The software is enabling a signal, hell the signal might even have a vague name, when a condition is met. The condition's name might not very clear.
Yes, a spec saying "when the car is undergoing a test then enabled the cheat mode to get past the test" would clearly implicate the developers.
But most likely it was: "when sig_x and sig_y and indicator_a are set, then set sig_Z to 1 in pattern P for n nanoseconds blah blah blah"
Someone knew what they were doing, and it probably went like this:
Engineer: We can't make this engine pass NOx tests.
Message goes up the chain to a certain decision making level, possibly the board. Marketing chimes in: We can't have this, we're already sending out teasers about our new urea-less engine technology, etc, etc.
Eventually a message comes down to fix it, in vague terms, entirely forgetting the original message that it's unfixable.
Engineers: struggle for ages.
In pub: Well, we could enable a special testing mode to pass the tests?
In work: Shall we do this -> up the chain. Original context is half forgotten. Approved.
Changes made. Software specs made. Timebomb implemented.