Hello, I'm looking for some help diagnosing the Red Triangle of Death on my 2005 Prius. The triangle came on a few weeks ago, and since then I’ve noticed a loss of acceleration power and the fan running loudly in the back. I tried running a test with Dr. Prius, but it wouldn’t complete because the battery was too cold or something. I decided to open up the battery pack and test all the cells individually, following the instructions in a video using a Harbor Freight battery tester. I recorded the results in a spreadsheet, and I’m not seeing anything that looks particularly bad, but maybe I’m interpreting it incorrectly. The lowest health status was 60%, and most were 85% to 100%. None of the internal resistances looked too high either. Would that be enough to trigger the triangle and cause these issues? My other thought is that the green corrosion on the bus bars might be causing problems, or maybe it’s a bad inverter cooling pump or dirty fan. thanks!! CELL, Health, Volt, Internal Res (mOhm) 1 83 6.5 6.23 2 100 7.65 4.9 3 100 7.73 4.88 4 100 6.72 5.25 5 100 7.65 4.86 6 60 7.23 7.29 7 100 7.66 4.87 8 85 6.39 6.16 9 100 7.78 5 10 100 7.77 4.92 11 100 7.62 5.31 12 100 7.06 5.59 13 100 7.86 4.96 14 100 7.87 4.96 15 100 7.66 4.92 16 100 7.87 5.02 17 100 6.92 5.27 18 100 7.42 5.11 19 100 7.67 5.01 20 100 6.61 5.2 21 100 7.72 5.04 22 100 7.7 5.11 23 100 7.39 5.11 24 100 7.66 4.98 25 100 7.68 5.06 26 100 7.1 5.18 27 100 7.51 5.31 28 89 6.35 6.02
^ This. May as well find out for what reasons (out of the couple hundred possible) the car has shown the triangle, before just starting to do things.
And I'm willing to look at those battery module voltage and capacity numbers, but my morning coffee hasn't kicked in so would prefer a frame of reference before going too deeply down this very familiar rabbit hole.
The voltages of the 28 modules are all over the place. A static reading of healthy battery modules should not vary by more than 0.10 volts of each other. My guess is that the trouble code that occurred was P0A80 " Replace Hybrid Battery Pack".
That's common if a failed battery pack sits for a while. As in this these numbers may be just self discharge data. As in modules that have self-discharged the most should be noted on the spread sheet to further the pack diagnosis. There's a dozen additional types of data points for evaluating a module/pack you can read about on PriusChat. Resting voltage isn't as good of a data point compared to module voltage loss after 2 minutes under load from a 50w/12v bulb. But this is just preliminary data... I'd like to see a charge and balance of the pack as well as a couple of deep cycling of the pack before drawing any conclusions beyond the replacement of failed or hot-running cells. All this and more is how you'll find the bad modules before they find you.
Holy bejesus. I can't believe not one person has been able to give you the answer you seek. In regard to the postings above, the OP stated: "The triangle came on a few weeks ago, and since then I’ve noticed a loss of acceleration power and the fan running loudly in the back." Which means the battery had probably not been sitting for months before getting his voltage readings. Yes, your voltages are all over the place. Yes, any module that is under ~7.2v is bad and anything that has drifted down to ~7.4ish will likely be bad very shortly. A module that can be salvaged and become reliable does not drop below 7.2 volts for months and very good ones will stay 7.6 or better for several months. All the ones in the 6's? Toss 'em. They will never be worth putting back in your car. The others that are less than 7.4 may or may not be salvageable, but with the number that you need to replace, it's not worth the risk.
..., cost, or effort. Mostly the effort. There is a lot of time and patience required. The hardest part in all this is having the quantity of replacement (years-old) modules that you can match to the existing (years-old) modules you currently have in your battery. The incoming modules need to have a very close capacity first and foremost to successfully rehabilitate a tired old battery pack.
OP didn't give vehicle history, specific error codes nor how long it had been sitting prior to triangle of death... But if the car was a daily driver it wouldn't have such terrible voltage numbers right away like that. There's a serious lack of information about what led to those bad numbers, but in my experience numbers like that are caused from the car going a long period of time without use. And you'd be throwing away perfectly good modules if you dumped the 6 volts without knowing the details of the pack's history. I've worked on modules that were healthy even though it had been down near 1 or 2 volts per module after sitting unused for years. And to your point, we should of explained that the symptoms OP is experiencing due to red triangle is the computers disabled hybrid system and left only the fan running at full blast while vehicle is running on ICE power alone.
Okay, so I do realize that I didn’t really give you enough information to adequately help me in my situation. So here’s the full story. This car has been in the family since 2005. It was daily driven by my mother and then my sisters for many years. It sat from sometime in 2023 until summer 2025, when I began to fix it up a bit. The 12-volt battery was bad, but surprisingly, the hybrid system didn’t give me any error codes, and I was able to daily drive it for about a month or more before the red triangle came on. The battery has been repaired by Prius Kings a few times. We have the lifetime warranty on it, but they’re so difficult to contact and so dodgy that I gave up on having them fix it and decided to try it myself. The red triangle came on and the car went into limp mode. I let it sit for about a month. I then tried to do a battery health test with Dr. Prius, which wouldn’t complete because the battery was too cold to complete the test (which I found weird because it wasn’t a particularly cold day). Anyways, when looking at the battery monitor values, I could see that some module banks were in the yellow and far lower than others for internal resistance (to be honest, I can’t remember exactly which ones or how many). But this was enough for me to decide I needed to open up the battery. I had watched two videos of people using the Viking 12V digital battery tester from Harbor Freight to evaluate these cells and use the state of health and internal resistance to determine which ones to replace. After doing this, I made the table I uploaded here and ended up replacing cells 1, 6, 8, and 28, as those were the ones that looked worst to me. I have since put it back in the car. Initially, it did not work, but I realized that it was because I didn’t have the safety lever pushed down correctly, which gave code P0A0D. I fixed that and cleared the code and tried to start a drive, but soon after, code P3000 came on. I tried clearing that, but it comes right back on. The monitor shows the internal resistances are much better, which is good, but I’m not sure if it’s struggling because the voltages are low from sitting for around 1.5–2 months at this point. Another thing that could be part of it is I didn’t put everything fully back together. I connected all the wires, the blower, the vent tubes, and the main top cover, but I skipped the two left-side covers/brackets because I didn’t know if I would be pulling the battery out or taking the covers off soon to test things. Do these have to be connected for proper grounding, or is it okay for me to leave them off for initial testing? I'm sure my battery testing would have been more useful if i had done it soon after i stopped driving the car. but now i cant get the car to recharge the batteries because its disabling the hybrid battery. What's the best next step for me? I really do appreciate you all taking the time to consider my issues and dealing with my ignorance
