Bad Water Pump or Spark Plugs?

Your 12 V battery is old enough that it is suspect.
The 12.4 reading is another possible indicator that it is going bad.

 

Nope! Need the codes. Everything else is total speculation. Let us know when you get the code reader. Hopefully, you made sure to order one that reads the Prius proprietary codes. Most code readers do not. The best option (and cheapest if you have an old Windows XP or Win7 32-bit laptop) is one of the Techstream and dongle kits available through Amazon or Ebay.

 

Start by checking tire air pressure.

 

See if the Inverter cooling pump is bad.

Check Inverter coolant pump:
Make the car ready and then open the hood and take the cap off the inverter coolant reservoir and with a flashlight look inside at the fluid it should look like its boiling which means the pump is working. If the red fluid is not moving at all the pump is bad.

Check the engine coolant:
That must be done by removing the radiator cap. Disregard the plastic overflow tank coolant level. The level in that tank is pretty meaningless. You must look inside the radiator itself for correct level check.

Take the black plastic cover off the top of the radiator. It has 5 screws easy way to remove screws is to snake your hand behind the screw and give the screw a little push as you unscrew it with a screwdriver. With cover off you can access the rad cap its on the left. With car off and engine cool open the cap the fluid should be at the top. If low or very low you have a bad ice water pump. The fluid level in mine has not lost a drop in 12 years.

You do not have to put the screw's back in that cover. The cover fits snugly and the hood traps it in when closed. My cover has been missing screws for about 10 years. I like to check all the fluids every Saturday and the loose cover makes it fast and easy to check rad level. Low coolant is the only hint you get that the ice water pump is leaking. They start weeping when going bad. if not addressed the engine will overheat and destroy the cyl head. You will never know till the check engine light comes on by then its too late. Many an overheated engine replaced on this forum. No temp gauge is ridiculous.

You have no engine temp gauge so it behooves you to be vigilant. Your wallet will thank you.

 

Possibly.
A weak 12 V battery can cause all kinds of erroneous codes to pop up....but there often is more than one and sometimes they come and go and change.

You should definitely check the voltage on the battery itself after sitting overnight and BEFORE starting the car.
Then again after READY.
Should be around 12.5 when "cold" and a bit over 14 when running.

 

You should NOT use the MFD for this reading......because some things must be "started" for it to work.

 

Maybe, maybe not. Get the three-digit info codes associated with P0A78 and P0A94. Depending on the info codes, you might get additional clues about the fault.

Mini VCI allows visibility to the info codes. On the screen which shows the DTC, look for the little blue cloud icon. Click on that and you will see info code detail.

 

that would be unusual. do you know the cars history?

 

The only thing the 12 V is suspect of is being a little low. A full charge with a battery charger will fix that.

But more likely, possibly not. See above re just being low.

While there might be a basis of truth to this wives tale, the reality is the battery will have to be pulled down into 8 – 10 Volt territory for this to even be a factor. Again, this line of enquiry is just a red herring.

And all this really tells you, yet again, you just need to charge up your battery.

Listen to the good advice from Patrick Wong. That is where you should direct your efforts.

One last thing, @ShenmueTacos, you haven't actually said what car is the subject of this thread, although you have said it is not the 2015 that is in your avatar and I presume it is a Gen 2 as you have posted in the Gen 2 forums. What year is this Prius with 110 kmiles?

 

Just stop it please.

It is virtually impossible to accurately tell what voltage the various computer boards are actually getting at the moment of boot up.
And this forum is FULL of all kinds of reports about multiple codes suddenly showing up and the cause being a 12 V battery going bad.

And it certainly is worthy advice to first charge up a battery that appears to be a bit low.......but if it IS a bit low, there is some reason and finding that reason often is important. The battery itself is but one of several possibilities.

Encouraging owners to chase codes pointing to systems that might require a LOT of time, money and effort to "fix" without FIRST being sure that their 12 V system is completely healthy, including loose connections. is just giving irresponsible advice.

Nothing I have said qualifies as an "old wives tale" or a "red herring" but what you have said certainly qualifies as BAD advice.

 

Make sure that all battery cables are secure including the negative cable where it bolts to the body. Measure voltage across the 12V battery when the Prius is READY. The voltage should be around 13.8VDC.

If you find the voltage is substantially different, that shows further evidence the DC/DC converter has failed. if the voltage is correct then I suggest that you verify the wiring harness connections via the troubleshooting offered by the factory repair manual.
techinfo.toyota.com

If the wiring harness is found to be good then you would have to replace the inverter.

The pdf you found is for a Nissan vehicle, hence it is not very useful or applicable to the Toyota Prius.

If you decide to replace the inverter you can look for a used unit which should cost in the low hundreds. If you have the dealer quote a new unit price, the price will be in the low thousands.

 

These are probably more useful for your Prius than the Altima PDF you found.

DTC P0A94/585 Diagnostic troubleshooting tree
DTC P0A94/589 Diagnostic troubleshooting tree