Hoping some Prius geniuses can help me get to the bottom of this mystery: I bought a 2002 Gen 1 Prius with 200K miles last week off Facebook for 2 grand. Before I bought it, I ran a Carfax report that showed it had lived its life in the Marin/Sonoma County area from 2002-2024, where the owner had it serviced 3 times a year at the local dealership (Hansen Toyota), and kept records of these services. The car is in mint condition, perfect interior, nearly perfect exterior. It's hard to believe this is a 24 year old vehicle. I trusted the maintenance history so I bought it. I drive it home from Fairfield,CA to San Francisco, driving it for about 1.5 hours without problem. The next morning, it was showing the feared Red Triangle of Death. I read that it can be caused by low oil and parking on an incline, so I put it on flat ground and gave it a quart. Triangle went away and ran perfectly. Later that day, I get a check engine light on for a faulty o2 sensor, which goes away on its own (I've ordered the piece and will fix soon). The next morning, the triangle comes back on. I detached and reattached the 12v battery and the light goes away. This morning, I go to start it (parked on flat ground) and the triangle comes back on. I reset the 12v like I did yesterday, but the light comes back after a few seconds running. I'm now at a loss for what to do. Consider: 12v battery has the number 7-26 on it, which I interpret at the manufacturing date, but I purchased the car on July 2nd, which would make it weird to have the battery made the day before. it was done at amp power batteries in Fairfield, california the hybrid battery seems to be new/refurbished January of this year, but the service shop isn't on Google Maps or anywhere online. The battery service didn't appear on the carfax report. I tried calling the number on the battery's sticker (what's app only??) but it's the weekend and they didn't answer. Unsure about a warranty on it. See photos. the person I bought it from sells cars professionally on Facebook and has a clean track record. He said he'd driven it around for a few months before selling it, and gave me new tires. there was no lights on when I test drove it It was successfully smogged on June 22, 2026. sometimes the red triangle comes on before starting (gen 1's have key ignitions) and prohibits the engine from starting. sometimes the engine starts and then the triangle comes on. what could be happening/what do I do next? Is this a computer issue? A 12v issue? a hybrid battery issue? some other issue altogether?
Important first step: can you read out any trouble codes? That 12V battery would not appear to be sourced from what's app, The HV battery isn't from them, apparently, as their site does't mention hybrid batteries. Problems with either could/would trigger the red triangle. The O2 sensor might trigger the red triangle. Later edit: Infinity Power, whoever they are, seems to be the source for that HV battery. Don't know a thing about them as a hybrid battery supplier. The HV battery would be high on my suspect list for your problem. My HV battery is this one: https://greentecauto.com/hybrid-battery/toyota/prius-hybrid-batteries/toyota-prius-2001-2003/toyota-prius-gen-1-new-cell/
Looks like a great car! Date on the 12V battery might be the warranty expiration. The first step is to read the engine diagnostic codes, not sure how you determined the O2 sensor was failing without them. Even a basic code reader can retrieve some generic HV battery codes. To get more detailed info might mean a trip to the Toyota dealership, or learn how to use Toyota Techstream and do it yourself. You could try to message Infinity Power about the HV battery through the WhatsApp app,, hopefully they'll get back to you.
I believe too much oil can trigger the triangle as well. Until you read codes, hard to say what your problem is. There's a process to help pinpoint an issue in which you see when the triangle triggers. Maybe someone else here can help with the details, I'm forgetful these days. You start with the codes cleared (check them first and write them down) and a well-charged 12v battery. Then, just turn the key on. Don't try to start it, just the key in the On position. Wait a couple of minutes to see whether the triangle sets. If it does, the issue is one thing (HV battery?). If it doesn't set, start the car but leave it in Park. Wait two minutes to see if the triangle sets. If it does, it's something else. If it doesn't, put it in gear, foot on the brake, and wait again. If it sets, it's a third thing, I think it's the electric motor. If it doesn't set within a few minutes, you're probably looking at some typical problems with the ICE - gas, spark, compression, air. Hopefully, you understand about the catalytic converters. Have you secured insurance on the car? I don't know that you can in California. Maybe liability, but not comp/collision/theft. If you do somehow, expect to have the car totaled with any claim, no matter how small. It's because CA is a CARB state (actually, the CARB state), and because these cats get stolen fast. You will need to always park in a secure location. When it's stolen, and it will be stolen if you leave it outside, CA requires that you replace it with an OEM cat or an aftermarket CARB-compliant cat, both of which are very expensive, as much or more than you paid for the car - or at least they have been for years. Because very few people buy them any more, maybe supply and demand has brought the price down - dealers dumping inventory. You have to be a long-time loyal policyholder for any insurance company to take that almost certain risk, and they'll look for any reason to total or cancel your coverage. They know that as soon as they pay for a replacement cat, it's just going to get stolen again. I sell used OEM cats to anyone silly enough to want one. Haven't sold one in more than a year. I talk most buyers out of it when they're not in CARB states. If they're in a CARB state and insist on buying, I recommend that they have two exhausts. Buy the OEM cat and repair your car, but also buy an aftermarket cat plus the exhaust section it's originally part of - from the exhaust manifold to the flange under the driver's rear seat. I have those as well. That entire section is just held on by 4 bolts and two rubber bands (about $20). Use the aftermarket exhaust as your daily driver, and whenever you have to get smogged, swap on your OEM exhaust and pass the test. Then, back on with the aftermarket. Any decent thief will recognize the aftermarket cat as worthless and not steal it. Even if they do, you just gotta buy another cheapo cat and repair it. You'll want to do the VAV hack on the vacuum line to make the check engine light go off, but that's easy. And it runs find without that, you just gotta see the CEL lit on your dash and won't know if another code sets since it's always on. When you decide it's not all worth that, consider selling the car across the state line, into a non-CARB state. The good news is that the car most likely has an OEM cat, so you can get a good chunk of your purchase price back - if you can navigate all the scammers trying to buy cats for way undervalue. It's tough. I've opted to hold mine hoping precious metal prices go up. probably should have sold last year - these metals spiked at the same time gold and silver did, but I was disappointed in the offers I got. Just not as much competition among the recyclers as there was in the the pandemic days.
