We have all heard the news by now: The National Highway Traffic Safety Administration has opened an investigation of 2004 and 2005 Prius based on 33 formal complaints from drivers who claim their Prius stalled without warning while driving at speeds from 35 to 65 MPH.

I decided to review the texts of these complaints and found some interesting facts from reading them. First of all, let’s keep in mind that we are dealing with 33 complaints generated by a fleet of 75,000 vehicles. This puts the complaint ratio at just under .05%! That said, any new car should not die unexpectedly in traffic. Obviously this could be hazardous and the Federal Government is right to investigate this and help with finding both a cause and solution. That is their job.

The Toyota Prius (Photograph courtesy of Toyota)

Based on my review of the reports, there appear to be four areas of concern and three corrective measures which have been taken by Toyota dealerships. The most likely causes of these shutdowns appear to be:

• That the vehicle was simply running too low of fuel! Several of the reports mention that the fuel gauge was showing two bars or less when the failure occurred. A couple of theories are floating around the internet on this, one that when the tank level (Remember that the Prius uses a bladder tank) falls below about two gallons the supply system can begin to suck air and cause the ICE to stall which then trips the hybrid drive system off. Another idea is that the software in the hybrid drive system was set to shut the system down when it calculated that the fuel tank would be empty based on input from the fuel level sensor. The evidence so far does much to bear this idea out. As far as I can tell vehicles that have had the latest software upgrades have not suffered a recurrence of the stalling problem. (It could also be true that the owners are no longer running them too low on fuel!)
• That some of these vehicles may have had a faulty throttle position sensor (TPS). Some of the reports described a situation where the throttle did not respond and the dealer reported back to the customer that the TPS had been replaced as a result. Of course, it is likely that these vehicles also received the latest software updates when they were brought back to the dealerships for repair.
• Several complaints mention difficulties with the Prius electronic shift. Its location close to the wheel and to kneecaps are mentioned, as is the apparent fact that too rapid movement of the lever may cause the hybrid control system not to respond to the current shift setting, such as when moving the lever from reverse to drive.
• As with so many other vehicles, it appears that the floor mat of a Prius can become wedged under the accelerator pedal just like it can in a conventional vehicle! This is known to cause a distinct loss of acceleration.

The solutions which appear to correct these issues are:

• Updated software for the hybrid drive system. I spoke with a Toyota dealership technician who indicated that vehicles are being updated on an ongoing basis.
• In a few of these cases a new TPS (and possibly the software update) appears to have solved the problem.
• Ensuring that the carpet doesn’t bunch up under the pedal also avoids certain performance complaints.

My conclusions at this time are:

• If the problem is indeed caused by faulty software it has likely been corrected as a result of the software upgrades being performed by the Toyota dealer network. I must emphasize that none of these vehicles have suffered a 2nd failure after being serviced.
• If the problem is caused by a lack of fuel then the issue becomes one of driver education rather than any fault with the hybrid drive system.