2008 Prius with C1241, C1310, C1378

sounds more like the hybrid battery

 

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From the description, it would appear that the HV battery is failing.

How many miles of the vehicle, and is the HV battery still the original OEM?
Did you noticed the HV battery fan in the rear passenger area running at an audible level?

There may be additional OBD2 codes (DTCs) stored in the ECUs (computers), and a toyota hybrid compatible scanner can retrieve them. Many prefer using a 'Mini-VCI' cable and techstream (dealer technician software) or a windows device. Some get by with using an OBD2 bluetooth adapter with an App like Dr Prius or Hybrid Assistant. Which scanner was used for the above DTCs?

Don't fall for the refurbished/rebuilt options, either a DIY repair, low mileage used, or install a NEW hv battery

 

Likely an OEM then, and could be a good candidate for DIY rebuilding, are you mechanically inclined and process tools?

You didn't miss anything, that was a basic limited scanner that wasn't hybrid compatible. Knowing the codes (DTCs) can be helpful.

Best to read and record the OBD2 codes before disconnecting the 12v, which will erase them until they are set and stored again.

Tell tale sign of a failed/failing HV battery.

FYI: you're posts are moderated until you've posted five times.

 

Start by grabbing a "mini-vci" cable ($20), then you will have access to the same OBD2 scanning software as the dealership. If that is too much, then pick up an OBD2 bluetooth adapter, but first research here before making a purchase : Hybrid battery diagnostic and repair tool for Toyota and Lexus

Best to go one step at a time. Do you plan on using the vehicle for anything?

If not, and while you wait for the mini-vci cable to arrive (pick a local seller for faster delivery), now might be a good time to learn how to get at and remove the HV battery, and clean the HV battery fan.

There are videos and threads on HV battery removal and fan cleaning, but following the Hybrid Automotive guide step by step will get you most of the way there : 2004 – 2009 Toyota Prius Plug-and-Play Car Harness Installation Instructions | Hybrid Automotive

Once you've removed the HV battery main cover, look for leaks/stains around the modules and underneath on the bottom pan. Check the HV battery ECU and associated connectors on the drivers end of the pack for corrosion, also get a visual on the level of corrosion around the voltage sense wires, busbars and nuts. If it looks bad, you may want to order a new harness for $60.

For others :

Here is the workup for DTC C1259 and DTC C1310 : https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/21avn/cic12595.pdf

Here is the workup for DTC C1241 : https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/21avn/cic12414.pdf

Here is the workup for DTC C1378 : https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/21avn/cic13784.pdf

 

Given your above description, the HV battery is failing (or has failed), scanning with a hybrid compatible OBD2 scanner/app is best.