Wednesday my beautiful P85 Model S became disabled, requiring a tow to Tesla Service. But by noon Thursday Tesla restored it. And therein lies a tale. On my way back home from a longer-than-usual day trip, thoroughly enjoying the Tesla driving experience, I stopped around 2 pm at a major-chain grocery store a couple miles from home. After loading goods into the frunk, I returned to the driver's seat, and the double-boop alarm immediately rang coinciding with a written warning saying "Car Needs Service - Please pull over safely". Since the car was still in Park, I thought that was a strange warning. Then I tried to put the car in Reverse. It refused. Same for Drive. Then a new warning came up: "12V battery low - Car may shut down unexpectedly". That alternated with the first. I took delivery Feb 23 of last year, it now has about 14.6k miles, and it had its annual service five weeks ago. I got out, got back in, and started over. Same results. Now I was convinced it would not move again without help. My next step: call the Tesla roadside assistance number via my cell phone. After reaching Andrew at Tesla HQ, I described the situation. As we spoke Andrew downloaded the log files. Pretty amazing; it's like the telemetry mission control collects from spacecraft. In a short time Andrew said "It looks like a problem with the drive inverter". That's pretty serious, since that's how DC is converted to AC to power the AC induction motor. Andrew walks me through shutting down the car then rebooting. Still doesn't fix anything. Later on he says it could be some sort of wiring issue, preventing the main battery from supplying power, explaining the drain on the 12V battery. I could understand diagnosing the car from 400 miles away in a few minutes is not easy. So Andrew says he's going to call the local Service Center, 12 miles away in Costa Mesa, and schedule a tow truck to come get the car. At 3 pm the tow truck arrives at my S. I'm fascinated with the process the tow truck driver uses to keep the 12V systems alive (take off the nose cone to access the 12V posts) then hook up the car (it seems like he attached straps to the struts of the front suspension). At 3:30 pm the Tesla is almost completely secured, so I leave for home using other arrangements. Late at night, I check on my car using the Tesla app, and discover it is charging at about 25 mph and 200V 39A and I confirm its location at the Service Center. I happened to be up at 5 am and checked again and found charge was complete at 272 miles of range. That is notable because a couple weeks ago it held a maximum 257 miles of range. 272 miles is what it could hold when new, last year. Next morning at 8:15 am I call the Costa Mesa Tesla Service Center for my car's status. Brandon of Tesla told me that it was all done. Great news! And we coordinated when that morning and where Tesla would return my car to me. But I wanted to know: what was wrong and what did they do? Brandon explained that the car has two "contactors" that allow the battery pack to power the car. He explains they're like really big relays. I surmise those make the "clunk, clunk" sound when one first activates the car. "We think one contactor got pitted." he says. Later I look it up and it's how relays get stuck: NAASCO Northeast Corp. - Article Any electrical arc between the contacts can cause wear and "pitting", increasing the electrical resistance at those damage points. Since I already estimated that the battery probably delivers up to 1000 A, that's a highly concentrated potential heat source, enough to fuse metal. So the car detected an electrical problem, due to a contactor stuck closed, and entered a safe mode. "So what was the fix?", I asked. "We replaced the battery pack," he answers. I'm floored. That explains why the range is 272 miles, just like new, but I'm aware of the estimated cost of a 85 kWh battery pack. The consensus I've read is that it must be at least $25,000, perhaps up to $33,000. That's an enormous cost, but it's also shows their commitment to Elon's bolstered battery warranty, covering virtually anything short of sabotage. I'm impressed that the decision on such a big expense was made so quickly. Then I ask, "I had my annual service last month, so was there a way to catch this problem then?" He answers "The contactors are inside the battery pack, so we couldn't look at them in a normal inspection." Since the battery pack is sealed, is electrically non-trivial, and is a structural component, that makes sense to me. One wouldn't risk disturbing the innards of a component so highly critical to the car without a good reason. So I reiterate to him how impressed I am that they swapped out the entire battery pack. He says they completed that by the end of the day. I encourage them to take the time that morning to check out the car, saying I'd rather they be thorough than rushed. I'm still impressed that in only a few work hours they swapped a 1200-lb component and fully restored a critically disabled car. Tesla covered the whole expense, and now I have a fresh battery pack! Interesting observations: I deduce it takes the Tesla Service Center an hour or so to swap a battery pack, perhaps less. My car probably arrived via tow truck sometime past 4 pm, then they needed time to diagnose it, so we were lucky (!) enough that a brand-new P85-compatible battery pack was on hand, so they worked from maybe 5 to 6 pm to swap it out, then test it, then charge it to full overnight. The coordination of the Palo Alto headquarters with the Costa Mesa Service center is pretty tight. From experience the Service Center's normal workload is busy enough, but they were able to promptly deal with my car. I called at about 2:20 pm and the car was returned by 11:30 am the next day. I could've asked for the car earlier in the morning. I appreciate the speed of that turnaround. I'm lucky this happened just miles from home. I wouldn't want to imagine if this occurred on a distant road trip. Then again a critical failure can occur in a car of any power train. Unanswered questions: What's to become of my old battery pack? Even if they replace the stuck contactor, surely they can't simply install it in a new Model S. Who knows if a few of those 7000 Panasonic 18650 batteries have failed. Does Tesla refurbish battery packs? Perhaps they'll take it apart for study, to learn what happens to a battery pack after 14.6k miles of use. Then maybe they'll reuse its components with that of others to create refurbished battery packs that they use for replacements like in my situation or for the announced Tesla "battery swap" stations. (One of you will have "my" pack!) How many brand-new battery packs do these Tesla Service Centers keep in stock? They probably need one of each of the 60 kWh and 85 kWh. Perhaps a third for a P85? My Questions to TMC members: Have any of you had your battery pack swapped due to a failure (not an upgrade)? Have any of you exercised the battery pack warranty?