Bill Sommerfeld's Weblog

Thursday September 29, 2005

And something resembling a root cause analysis. The Prius saga continues. Toyota sent the NHTSA a complete reply on August 26th.

The meat is in Responses 8 and 12. It appears that Toyota released a patch in October 2004 which fixed a firmware bug - apparently the stall occurred when the firmware thought the engine wasn't taking in enough air, but the "not enough air" threshold was set too high. Some of the details are in attachments that were not made public, but it's now clear that they're confident they understand the cause of the stall:

"Under certain circumstances, the engine ECM incorrectly determines that the gas engine is experiencing a failure to start when the engine intake air volume is lower than the ECM's programming criteria. In this condition, the gasoline engine will not start (because the ECM believes it cannot) and the vehicle will go into a fail-safe mode of electric-only operation. In conjunction with the ECM misjudgement, the warning lights ... will be illuminated when this occurs."

and there are two relevant fixes. The first one was released as part of "Special Service Campaign 40A" in October 2003:

"Due to a programming error, if the vehicle is restarted in the "fail-safe" mode, a secondary condition may occur where the vehicle transmission may not operate smoothly."

Subsequently, they released TSB EG047-04:

"Toyota discovered a software error within the engine intake air volume criteria ... Toyota developed a revised software version and introduced this software along with reprogramming methodology in a TSB in the middle of October 2004"

What's perhaps a bit strange is that the first bug and a third unrelated (and seemingly trivial) defect were the subject of two different "special service campaigns" where they actively asjed customers to bring in their cars for a firmware upgrade, but the seemingly more critical bug (the apparent proximate cause of the stalls) is only subject to a TSB, which appears to be a "fix it if the customer complains" reactive patch. If I buy a Prius I guess I'll feel obligated to check for TSB's on a regular basis...

(2005-09-29 15:12:04.0) Permalink Comments [2]

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