This is not a question but rather an account of problem resolution that Car Talk readers may find interesting.
Drive-by-wire throttle system crashes are mentioned occasionally in threads on different forums, but few have posted their resolutions.
Drive-by-wire throttle systems are great for reducing manufacturing costs and parts counts on cars with cruise control, traction control, stability control, etc. They can be a pain, however, to diagnose when they fail.
All functions in an EML have redundant components, and the system is designed to fail closed when it detects ANY problem. This is because manufacturers live in fear of charges of unintended acceleration.
On a BMW, a soft EML crash gives you a limp-home mode in which you have about a 40 mph top speed. More commonly, however, the EML crashes completely which gives you a fluttering throttle butterfly in a nearly closed position, 1100 RPM and zero power. Shutting off and restarting the engine resets the computer and generally allows you to drive normally again.
Solutions I have found posted included failed throttle body, failed throttle pedal, and failed wiring harness connections making it appear to the computer that the throttle or throttle body had failed.
In MY case, however, ('04 325iT, 140k mi) I got a P16A7 code, which is defined as “MAF signal implausible”.
The reason my MAF signal was implausible was because my bank 2 catalytic converter was completely plugged up, and my bank 1 catalytic converter was not too good. The only reason that cyl 4 through 6 were running at all was because their exhaust was going up through the secondary air passage and coming out in the front exhaust manifold.
When the computer detected that the MAF was not reporting nearly enough air flow for the throttle opening and RPM, it shut down the drive-by-wire throttle system. When that happened, I just turned the key to the off position for a half second and then back on. The car bump started and continued to drive as if nothing had happened, other than the Service Engine Soon and sometimes the EML lights illuminated.
It took me a while to figure this out because my test for an obstructed exhaust is to put my hand next to the exhaust opening when the car is idling. If I can feel individual bursts of exhaust, it is OK. This does not work when you have two cats feeding a single exhaust. I did not identify the problem for certain until I dropped the exhaust pipes and put my hand at the opening of the downpipes, below the cats. Then it was obvious that one was blocked.