P0607 code, cruise control light flashing, no other symptoms or lights on dash

That's a real code, but it's not an HV battery code. It's a code from the engine control module. There shouldn't be answers all over the place about it; it's a failed ECM self test, and unless you convince yourself it was a pure fluke and has never happened again, the indicated solution is to replace the ECM. (The manual first recommends that you check for any codes other than the self test codes, and diagnose those if you have them, then see if fixing those resolved the issue; if not, or if you don't have any others, it goes right to "replace ECM".)

There's a large school of thought here that a low-enough 12-volt battery supply could produce all kinds of ECU malfunctions. There is less consensus about what "low-enough" and "all kinds" really mean, and I'm personally in the camp that doesn't think it's the explanation as much of the time as some others do.

But there's a variation of the "if you have to ask ..." saying that might apply here. If you've got an ECU telling you it failed a self check, and you feel like you've got any reason to question your 12 volt battery, well, have you checked that out yet? I don't know for sure if it'll turn out to be your culprit, but you'll be happy if it does, right?

 

It might be related. It might not. One way to tell would be get a mini VCI cable from amazon or ebay along with a bootleg copy of Techstream (Toyota's diagnostic software) and install it on an old windows laptop. That would give you the ability to check "freeze frame data", a snapshot that the ecu took when it set the code. It usually includes things like battery or ignition voltage.

That said, testing and replacing the 12V as needed should be normal maintenance.
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I had same indications with P0571, P0607 and cruise light flashing. Replacing brake switch did the job.

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