The reports cite comment cards or labeling within the powertrain control module programming as evidence. There are blocks of logic labeled "Test" and "Degrade" and other cryptic names.
True. Those labels exist in the programming. The falsehood is that those chunks of programming exist to "game" fuel economy and emissions regulations.
|Utah, I-70, centered on mile marker 110. Walking out is not a good option.|
The worst engine failure mode is to force the occupants to walk-home. That is because the failure might occur in a place that is inherently dangerous.
A vast amount of programming is invested in creating functional vehicle "limp-home" modes.
How many times have you had an engine light go on?
It is often a sensor that either failed (often from a wire breaking) or it is producing extremely unlikely outputs, i.e., acting flakey.
|I-94/I-96 interchange. This used to be vibrant, residential neighborhoods. Now it is a wasteland. You do not want to have to walk-home from here.|
A sensor that commonly fails is the O2 sensor. This sensor is mounted on the exhaust system where it is doused with muddy water, hit with all kinds of trash thrown up by the tires and occasionally dragged through snow and chunks of ice. The exhaust system sways during its normal operation. It gets hotter than the hubs of Hades during operation. It is a very hostile environment for an electrical sensor.
The O2 sensor monitors the presences of free oxygen (O2) in the exhaust stream. The computer uses this information to continuously adjust the amount of fuel to keep the process exactly balanced on the razor's edge of stoichiometric perfection. That is, every carbon has two oxygen atoms and every two hydrogen also has one oxygen atom.
When the O2 sensor fails, the engine control computer references a table or map that estimates how much air the engine is pumping at a given RPM, manifold absolute pressure and throttle position. It then guesses how much fuel to inject based on that historical information.
The computer always guesses rich...by design. Suppose you were running "pure gas" before your O2 sensor crapped out and then started using fuel with 10% alcohol. A gallon of fuel has less carbon and hydrogen and would cause the engine to run lean...which might result in burning holes through the pistons....back to walk home.
The computer is also likely to use degraded transmission shift points. It is more important to get the occupants to safety than to optimize fuel economy and emissions. Would the vehicle pass "the test" in this condition? Probably not. Is that important? Not to the occupants who just want to get home.
Not just the O2 sensor
It is my belief that backup programming is in place for every sensor.
It is also my belief that backup programming is in place for every combination of two (and sometimes more) sensors failing. For example, an engine that overheats (perhaps due to a punctured radiator) can fry multiple sensors or take them well outside the envelop where their calibrations are valid. The computer will not force the occupants into walk-home mode. The engine will eventually seize up leaving no other options....but the computer will not.
Personally, I think fault tolerant programming is a marvelous concept. I wish it was used in the social sciences.