Howdy, so my 2005 prius is coming up on 300k miles on the engine. It's been a real trooper. But suddenly this summer, it gave me the dreaded red triangle. I didn't have the time or energy to troubleshoot it or the money to pay someone else to, so I let it sit for a few months, and now I've been trying to figure out the problem. I got a basic OBD scanner, and the codes I'm getting are: P0037, P0420, P1121, P1150, P1116, P3190, P0301. I also took it in for a basic oil change and fluid top-up two weeks ago to rule out any issues from sitting too long (it was due for the oil change anyway). Known issues: the engine has been burning oil and needing regular top-ups for the last couple years, which seems normal for the age of this engine. I did a DIY catalytic converter replacement in early 2021, and it has been working fine since. Non-OEM parts for both the cat and the O2 sensor, which was cut when the cat was stolen. The replacement was not my best ever welding job but has held up ok. From my research, and here's where I could really use some help because to be frank I know just a little more than shit about cars, the codes I'm getting fall into two main buckets: A: Emissions/cat, P0037/P0420 B: coolant system, P1121/P1150/P1116 I've been focusing so far on B, the coolant system, because this seems most likely the cause of the red triangle and potential cause of serious damage. In my research on the forum this combination of codes seemed likely to be the result of air bubbles in the coolant system somewhere. I followed a youtube tutorial on 'burping' the coolant system, but honestly it doesn't seem like there were/are any air bubbles. As we speak I'm waiting for the engine to cool so I can see if it needs any more fluid, but from what I can tell there are no leaks, no air bubbles, the temperature is correct (OBD sensor read it as 85 C once engine heated up), the cabin heater is working and I can't hear any glopping sounds behind the dash. Seems like fluid is circulating since it's heating and nothing is overheating but idk how to check for a clog anywhere. So, what can I try next? Replace the 3-way valve? Could my A cluster problems be causing the red triangle, and will they cause further damage to drive on? If I have an exhaust leak, how can I tell where it is? Research seems to indicate I should address A cluster problems before B, but I'm not sure why.
None of those are hybrid-system codes (the ones that light the red triangle). Most often, this means your OBD scanner is too basic, and can't show you the hybrid-system codes from a Prius. So we're still sort of guessing blind at why the triangle lights up. I mean, it could be as simple as a P0A0F code on account of the P3190, but that's still a guess without access to a scan tool that will show the hybrid-system codes.
If I take it to an autozone or jiffylube, will their scanners give enough detail? or do I need to take it to a dealership you think?
It would be iffy at an autozone or jiffylube. Doesn't have to be Techstream (the software the dealer would use) but does need to be a scan tool with comparable functionality, like the Autel frequently mentioned around here.
I have an 07 Prius. More than amonth ago I went out to start it and the dash lit up with all kinds of lights along with the triangle. I tried to put it in drive but it only went to neutral, same with trying to put it reverse. There is a light that says "BRAKE" on left side and on right side it has the red triangle, brake icon with !, ABS,VSC, and CHECK engine icon. I had it towed to the Toyota dealer for diagnosis. They said the code AOFOA, or something on that order, means that it needs a new hybrid battery. I balked at the $4,789 price and had it towed home. After watching videos I took the battery out and set it on the workbench and tested each battery module. They all were 7.5v or better. I bought a charger that would discharge and charge four modules at a time. After three weeks I balanced them and put it back into the car along with a freshly charged 12v battery that is two months old. Turned the car on and nothing has changed. I need a better scanner because the MOTO POWER MO60033 that I have says no codes are stored in system. I saw where someone posted that pushing the emergency brake on and off a few times fixed his not going into drive problem. It didn't fix mine. I'm thinking about replacing the brake switch behind the brake pedal but not sure yet. Anyone that can help me would be appreciated. Also what kind of scanner can I buy that will give me codes in the system?
I'm sorry I meant to upload a picture of what Toyota found with their scanner. I'll try to upload it now.
Alright, so Autel scanner came in, here are the codes it gave me: Hybrid Control P3000—battery control system Transmission Control C2318—low voltage error (power supply malfunction) ABS/VSC/TRAC C1241—Low battery positive voltage or abnormally high battery positive voltage C1253—Hydro-booster pump motor relay C1310—Hv system malfunction Engine P0304—cylinder 4 misfire detected Hv battery P0A80—replace hybrid battery pack AC B1421—solar sensor circuit (passenger side) B1441—Air mix damper control servomotor circuit (passenger side) How screwed am I? Is this fixable as a DIYer with credentials from YouTube University? The car has almost 300k miles and isn't in great shape otherwise, not to mention it now has a growing mold issue since it's been sitting and we're now fully in the rainy season. That being said I can't afford to buy a car. So it will be e-biking and bussing for me if I can't fix this, until I get a job that helps me afford a new vehicle.
You could be less screwed than you think: the P3000 code may be there because of the P0A80, and the C1310 because of the P3000. The C2318 and C1241 might both be related to some low-12-volt power event, and it might even be possible that event was a consequence of a traction-battery problem, if the DC/DC converter output got curtailed because of it. B1421 isn't a problem, just a code provided so you can tell if the dash solar sensor is working (if it's working right, you have B1421 when it's dim or dark out, and you don't have it when it's bright out). The C1253 might be related to the C1241. The B1441 you might not care about much, until after other things are sorted. So there are some things that need attention and troubleshooting, but maybe not as many different things as it seems.
Alright, that's genuinely very reassuring. Im guessing what I need to do then is test and replace some of the hybrid battery modules? is this a good overview to start with? or is there a more recent thread you're aware of I can look to for resources? Gen II Prius Individual Battery Module Replacement | PriusChat
the brake switch is fine if the brake lites come on. start with a free 12 volt load test at most auto parts stores.
Jan 3,2026- I saw a post about the Hybrid battery voltage/temp sensing ECU sometimes burns the connector plug out so I took the ECU out and sure enough not only was the connector burnt but the pc board inside was also burnt. I ordered a new cable with the temp sensor wires and the ECU all together. After installing and charging the 12 volt battery and having all 28 modules balanced at 8.0v to 8.1v I put the hybrid battery back in and turned it on. Nothing changed. I ordered an Autel MK900-BT scanner and cleared the codes and reran diagnostics. The only code that came up is P3004 but the car still would not start and all usual warning lights are still on. Through more research I have read that I need to have the ECU relearn the VIN. I thought that the Autel scanner that I bought would do this but it does not have the right programs so I am probably going to send it back. I have been reading about the Techstream mini VCI that can do the relearn process. Here is the problem, I have read about some of the Techstream software sold with the VCI is fake or out of date. How can I be sure to get the correct software and VCI? Anyone know where I can find videos on the Techstream install and use of it? I have seen many but I do not know what is con job and what is not.
The hybrid battery ECU does not need to learn the VIN. It sounds like you made good progress by replacing the voltage sensing wire harness and hybrid battery ECU. Now, you need to concentrate on the P3004 code that the car has. Does the Autel scanner that you bought show you the freeze frame data for the P3004 code? If it does, there will be an INF detail code that will help narrow down the troubleshooting process. You said that the car does not start. I am also assuming that the car does not go into READY. Is that correct.
