If you are not an engineer, you may not know what a Wiebull reliability analysis is. That’s a statistical technique used to estimate when a particular part will fail based on available data. In October, Plug In America completed a survey of 327 Tesla Model S owners with 2012 and 2013 cars. It asked them if their drive unit had been replaced and, if so, at what mileage.
Using the data collected by Plug In America, Matthew Klippenstein or Green Car Reports ran a Weibull reliability analysis and found that two thirds of all drive units in those early cars will likely need to be replaced by the time the cars accumulate 60,000 miles. That’s a disturbing statistic. The results are plotted in the graph shown above.
When Klippenstein reviewed the Plug In America data, he found a few transcription errors that might have adversely affected the reliability analysis. So he corrected those errors and ran the analysis again using Reliasoft, a free online tool. This time, the average predicted failure time rose from 57,000 miles to nearly 60,000 miles. He replotted his findings and came up with this chart:
The old expression “Figures lie and liars figure” applies to all statistics. In order for the results of the analysis to be relevant, the sample size has to be 1% or more of the total population and the participants must be randomly selected. In this case, 327 respondents is more than 1% of all 2012 and 2013 Model S owners. However, the data was collected from people who were willing to respond to the Plug In America survey, a factor that should be kept in mind when assessing the merits of the conclusion.
Before publishing its story, Green Car Reports asked several pertinent questions of Tesla, as follows:
- How many motors has Tesla repaired or replaced in 2012 and 2013 Model S cars to date?
- What percentage of total cars does that represent?
- What does the company’s own reliability data indicate about the percentage of cars that will require motor replacements over the life of the vehicles?
- Musk said in November that reliability had doubled; how confident is Tesla that it has now fully resolved the reliability issues with earlier motors?
- What is the company’s projection for failure rate in the motor(s) of current production cars?
- What would Tesla Motors say to owners (and future used-car buyers) of the early cars regarding the reliability of the motors?
Tesla declined to give specific answers. Instead, it submitted this general statement: “Close communication with our customers enables Tesla to receive input, proactively address issues, and quickly fix problems. Over-the-air software updates allow Tesla to diagnose and fix most bugs without the need to come in for service. In instances when hardware needs to be fixed, we strive to make it painless.”
Indeed, when reports of problems with early drive units first surfaced a few years ago, Tesla stepped up by increasing its warranty coverage of the drive units to 8 years and unlimited miles. To date, there are no known instances where a Tesla owner has had to pay to repair a faulty drive unit. The reliability analysis does show that that the other components in the Tesla drive train such as inverters and electronic controllers have a predicted life expectancy of 1,000,000 miles. Elon Musk has said recently that the quality of its drive units has improved significantly of late and that the company is targeting the 1,000,000 mile mark for all driveline components.
Still, the situation was enough to convince Consumer Reports to delete the Model S from its list of recommended cars, citing worse than average reliability. For a company that relies almost exclusively on word of mouth to sell its cars, the number of failures is a matter for concern. As time goes on, Tesla will start entering the used car market in significant numbers. If second and third owners start paying hefty repair costs, that will damage Tesla’s otherwise excellent reputation. Some worry that the cost of so many drive unit replacements may also be a drag on future profits.
For now, Tesla owners say they are happier with their dealer service than the owners of any other cars. Much of that has to do with the fact that Tesla, so far, has fixed most major issues for free. But how long will that policy last?