The Red Triangle was only the beginning

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When was the last time you checked the engine oil?

It sounds like you have a failing HV battery. Next step would be to make a plan to either fix it yourself, or pay someone else to do it for you. After making a plan, you should visit each of the auto parts stores (even within the same chain) and get a free OBD2 scan. Once you know, and have recorded the OBD2 codes (DTCs), post them here for additional guidance. Which part of Norcal/Socal are you currently near?

FYI : you're moderated until you've posted 5 times.

 

It's very possible that the traction battery has a problem. As @SFO pointed out, you need to get the codes read. Make sure they use a scanner that's sophisticated enough to talk to all the ECUs in a Prius. Most are not.

But I'm not sure the battery actually has a problem if the warning light is just coming on when you brake and then goes back off. That's the car screaming at you to check the oil and top it off. But it may be too late for that and the damage is already done. Not going over 25 mph and the battery fan running loudly suggests that the electric motor is trying to compensate for a partly seized engine. On the other hand, if it's original, that's a pretty old battery and it's already performed beyond the typical lifespan.

 

Oil

 

I second the recommendation to get the trouble codes read.

Additionally ...

... I would avoid going back to that mechanic with Prius questions.

When the traction battery fails, a Prius will not "become a traditional gas car." It will become stationary.

That's a fairly basic thing for a mechanic to know about the car.

 

For whatever it is worth, that mechanic's description isn't strictly correct for Honda Civic hybrids either. Yes, there is an auxiliary motor starter, so the gas motor can be started without the electric motor if the IMA is dead, but under many driving conditions nothing is charging the 12V, and it is possible to run the 12V down to the point the car stops. So while the HCH "sort of works" without the IMA, it is definitely not like a "traditional gas car". The closest analogy would be a gas car with a failing alternator, and who wants to drive that?

 

Highly unlikely. Replacing the ancient 12v may be a good idea, but only after you've sorted out the HV battery issue.

Both of the above (HV battery fan running on high, and 25 mph "turtle mode") are symptoms of a failing HV battery, if you plan on driving 350+ miles north in the current degraded state, you should drive slower, and don't stress the HV battery by braking or accelerating quickly. Think about taking the 101 instead of the I-5, as the grapevine may be too much for the HV battery to handle.

Do you (or someone you know) have basic tools and a DIY attitude, or will you be paying to repair/replace the HV battery?

 

The first two are helpful in a well-now-you-know-for-sure sort of way. C1259 and C1310 are codes the brake ECU sets to tell you that it has been alerted to codes in the hybrid vehicle ECU.

So now, if the scan tool you are using is not showing you codes from the hybrid vehicle ECU, you know for sure you need a better scan tool to retrieve the codes you now know are there.

 

Unlikely that anyone would recommend buying a 'reconditioned' or 'refurbished' HV battery, which is another way of saying "used". Most of the $350-$1500+ HV battery swap services won't last any length of time and you will be right back where you started from. Many here will recommend a NEW oem battery from the dealer (around $1800 with self install), or one of the NEW batteries linked below :

$1990+tax+delivery for a NEW lithium replacement : https://Projectlithium.com/

$1600 total, which includes delivery for a new NiMH replacement : NewPriusBatteries.com | Home

Let me know if you need any assistance with the install, as I'm just across the bridge.

Have you (or your mechanic) ruled out swapping the failed modules and "balancing" the existing pack?