Help! Prius throwing C1241 and C1300 codes despite new HV and 12V batteries

Hey all,

I just installed a new hybrid battery (brand new OEM) a couple weeks ago after the fire from my previous thread. Everything seemed to be going fine, then I got the red triangle of death with VSC and ABS lights so I pulled over. It initially said, "There is a problem with the transmission P lock mechanism. Park your car on a flat surface and fully apply the parking brake." Then it never said that again. Ran the codes on Dr. Prius and got P3102 and P3108. Then I ran Techstream and got only C1241 and C1300. The car still drives. The Dr. Prius codes haven't come back.

My 12V was reading low, in the 11s, and it was 4 years old so I replaced that. The C1241 and C1300 codes remain. I can't find any successful solutions to these nonspecific codes on this forum, as most suggest to replace the 12V which I just did today with an OEM brand new battery that reads >12.4V.

The car is also acting funny, the dash dimmer lights no longer work and they stay at a constant brightness, they also don't dim when my headlights are on. When I pull up the Prius' diagnostic codes, "ADIM/TCAN" swaps between DIM and bright repeatedly. When I go into accessory mode there are no dash lights in the car, but the main screen comes on. It's very strange.

Has anyone successfully fixed something like this? Could it be the engine ECU? I don't want to break off all the vents again removing them to get into the dash (they're extremely brittle from being in the sun all their life) for something that may not work.

 

The new 12V reads 12.75V on my multimeter brand new from the dealer with the car never ran.

The previous 4 year old 12V reads 12.38V after it sat for 3 days. That one is no longer in the car.

 

Thank you. My sub-code is 81 according to Techstream.

I found that guide on here previously and I've went through #1 (no HV codes), #2 (brand new 12V installed and reading 12.75V before ever being used with date of manufacture last month), and #3 (plugged in my wife's working brake control power supply tonight with no change in the codes).

#4 and #5 are somewhat confusing to me, as I don't understand how to check the voltages for each individual wires. I can't find any videos of this either. My multimeters metal prongs are way too large to fit in between those tiny wires. Any tips for checking this? I was debating on just replacing the skid control ECU with a working used one because I have no idea how to check this.

 

Noted. I compared my Prius' lights with my wife's gen 2 and actually my lights are working properly. The dimmer only turns on when the headlights are on and the MFD doesn't come on in accessory mode in either car, and hers has no codes and works perfectly.

So I'd like to redact that part of my entry paragraph and note that I think the brand new 12V is completely fine. It read 12.75V without driving the car.

 

I've ruled out:

• Brake control power supply assembly (tested with my wife's working one that isn't throwing any codes, I will also use the back probe on this when I get it but I'm not hopeful as I put a known working one in there with no success)
• Hybrid control system (no HV codes and a brand new OEM hybrid battery, the decision tree says to skip if "DTC is not output")
• 12V

Still on the differential:

• ABS No.1 relay
  
• Harness and connector
• Brake control power supply assembly

My next goal is to take apart the dash to get to the wire harness and connector. I believe that's on the driver's side and I have to take off a couple air vents to get to them. Might have to wait until this weekend as I work above full time as a surgery resident.

Yep the 12V was just tested by me at 12.75V. I can drive it first and check it again if you think that would be beneficial. This was undriven, straight from the dealer.

Perfect thank you! Couldn't find that info online with what I was searching. I'll buy that at Harbor Freight. Because I am probe illiterate, I just put the red probe into the wires I'm testing and the black probe on something metal in the car, correct?

 

Big Update!

I went to the junkyard and got a skid control ECU out of a wrecked but overall very beautiful condition 2007 Prius for super cheap, like $25. I didn't want to pop off the dash to try it in my car, so I snuck my hand in above the driver's side gas pedal and unhooked the 4 connections into my skid control ECU and into the "new" junkyard one. It was quite hard to get these unhooked but thankfully my hands are tiny.

Immediately codes C1241 and C1300 disappeared, but they were instead replaced with C1256 (Accumulator low pressure) and C1391 (Abnormal leak of ACC Press). I thought these codes may be due to my lack of calibration, so I pressed on with Techstream. I tried multiple online guides for this and none of them were detailed enough for my lack of knowledge, but then I found an amazing youtube video by one of our Priuschat members Jo that detailed exactly how to calibrate it step by step.

The video is here:

I reset the battery again and all the codes are gone now! I'll need to officially install this because I was too afraid to take off the dash but this is the first time I've ever not seen these codes at startup immediately after reset. There are no videos of how to install a skid control ECU so I'll try to make one.

 

Fixed! Thanks for the heads up. Hopefully this post can help someone else with these codes.