Jason Levine, executive director of the Center for Auto Safety, said that a car in limp-home mode may be better than a full stall, but still puts drivers in a "freaky" situation. "You have gone to a large golf cart," he said.
Levine and others compare Toyota's software fixes to Apple's secret modification of software that controls the iPhone, slowing down the device as the battery ages and loses its ability to hold a full charge. The Securities and Exchange Commission and the Justice Department said in January they were launching investigations into Apple's practices.
In many cases, the inverters fail when drivers step hard on the accelerator or brake hard, which subjects the inverter to high loads. In its recall notice to owners, Toyota suggested drivers should temporarily take it easy on their Priuses: "Until the remedy is performed, drivers should avoid placing a high load on the hybrid system by avoiding full throttle application when possible."
The company has been struggling with the overheating problem in the Prius as far back as May 2011, according to a "defect information report" that it filed with federal safety regulators in 2014. Its engineers examined the possibility that solder joints were cracking, a result of "excessive thermal stress."
In following years, engineers found cracks in the solder, but could not find a problem in its manufacturing process. Later, the report said, the cracks were not turning up in the wagon version of the vehicle, the Prius V, which has the same inverter. By 2014, Toyota finally settled on changing the software in the inverter and engine controller. It told federal regulators it would sent owners notices by 2015.
The overheating occurs in special electronic devices, known as "insulated gate bipolar transistors," or IGBTs, which boost voltage and convert the DC power from the battery to AC power.
Sponsored Video Stories from LifeZette
Heath Hofmann, an electrical engineering professor at the University of Michigan and an expert in hybrid vehicle power systems, describes the transistors as high-power switches that turn on and off thousands of times per second. The auto industry is trying to find a substitute for the silicon transistors, but so far continues to use IGBTs. It is likely the transistor loads are at the heart of Toyota's problem, he said, based on the defect information reports.
"It strongly suggests that they did something to change the vehicle's overall power management system," Hofmann said after looking at some of the Toyota recall documents. "The likely thing they did was reduce the power running through the inverter and the motor generator, particularly if they are having problems in high power demand situations."
Michael Pecht, a University of Maryland professor who founded the school's Center for Advanced Life Cycle Engineering, which focuses on electronics reliability, also reviewed the Toyota recall documents. "Clearly, they are saying the problem is in the IGBT, but they couldn't find the cause for two years," he said.
Pecht said the software fix to reduce temperatures in the inverters may help, but "it is just going to delay when the failure occurs. My gut is that the software fix saves money. This is really serious. The inverters need to be replaced."