Problem with the transmission 'P' lock mechanism

My daughter has owned this Prius (2007 w/285 kmi) for 5-6 years now. I got this error message this morning. I've read a bunch of threads on here and it seems like an old battery can be the issue. Here's the sequence of events that led to me getting it:

- **** Last night ****
- I started the car to warm up the interior since it was cold out.
- Went back into the house.
- Came out 5 minutes later and the engine was off but the headlights were on very brightly
- Drove 15 miles home
- **** This morning ****
- Drove 2 miles to Autozone to check a CEL
- Got in car and pressed the power button without foot on brake
- Plugged in code reader and got P0420 read (I know how to handle this)
- Locked car and walked back into store to return code reader (forgot to power off the car)
- Returned to car, stepped on brake pedal, and pressed power button
- Received Red Triangle and error message

I'm theorizing that leaving the car on sucked enough life from the battery to cause the problem. What do y'all think?

 

I wasn't in the store long at all but it is possible I pressed the button twice. Seems I heard the water pump going.

 

P0420 has been a nemesis of mine over the years in a half dozen or so cars. New O2 sensors have always solved it for me. You don't want to let it go too long or you run the risk of damaging your catalytic converter. If the upstream sensor is bad you can wreck the converter. I don't think the downstream one is much of a problem. Unfortunately P0420 doesn't tell you which sensor is the culprit. I once had a Ford Taurus with a V6 and dual exhaust so there were 4 sensors. It threw a code that pointed to one of the sensors so I replaced it. Then it threw a code for the other on the same exhaust side so I replaced that one. Then it threw a code that pointed to a sensor on the other side so then I just said funk it and replaced both. No more issues. I happened to have a second Taurus and a few months later I got the same first code. this time I replaced all four sensors and that was the end of it. Now I just replace all of them when I get the code. This was the first time I didn't replace them both. I was gonna but the code stayed away so I thought I got off easy.

This car has had this code for years too. First thing I did was replace the downstream O2 sensor since it only takes 10-15 minutes and it's the cheaper of the two. I cleared the code and it stayed away for about a month. I had actually purchased both sensors but only had time to replace the easy one and when the code didn't come back immediately I returned the the other sensor. The code came back on Sunday so I ordered the upstream sensor from amazon again. It's a little more work but I had also purchased the PCV valve and this is one of them jobs where if you're gonna do all that work you may as well replace everything else that's in there.

 

FWIW I replaced the upstream O2 sensor yesterday. The odd thing is the CEL was off when I fired it up to get it into the garage. Daughter said the shop she used for oil changes would clear the CEL and it would come back on and stay on shortly afterward. I guess replacing the downstream sensor helped but didn't fix it. We'll see if replacing the upstream sensor puts an end to this for another couple hundred thousand miles or til death do us part.