2008 Prius "Park Button" Not Working

The Park Button won't engage park (and therefore, disengage?), and when I shift into Reverse or Drive, the transmission almost immediately shifts back into Neutral (as indicated on the digital gear indicator).

About 10,000 miles ago I drained and refilled the transmission twice with Toyota WS trans fluid. No leaks.

No odd dash lights are illuminated.

What is the problem?

 

Replaced the 12V battery in March, though haven't measured voltage since this Parking Button problem started yesterday.

 

I have simple OBD2 reader, will this read the "C" codes you mentioned?

 

True, it's pushing 100 degrees here so I'll check in the am and report back here.

 

Update:
My OBD2 reader didn't indicate any codes.

Today the parking button is functioning 100% correctly, and the transmission is shifting into gear and staying in gear as it should.

WTH?

 

I followed a YouTube video and checked the 12V battery...Checked out great.

I am baffled, and open to more ideas on the cause and how to permanently resolve this "phantom issue".

My wife is about to drive the Prius 100 miles tonight, so she will give me a report.

 

When my wife tried to leave for her 100 mile drive the Park button and transmission acted up again, same weirdness as before.

At this point I plan to make an appointment at the nearest Toyota dealership for diagnosis.

Thanks to all for your input so far. I will report back here after I learn something from the dealership.

 

Update: Despite the fact the engine battery checked out as okay, I replaced it with new (Napa brand with 3-year warranty) this morning. Now all systems are working...The battery (Exide brand with 4-year warranty) I replaced lasted only 3 months, and the one before that (Exide brand with 4-year warranty) lasted12 months.

Again, no CELs are illuminated.

I canceled the appointment for my Prius at the local Toyota dealer.

Any ideas why my Prius seems to frequently need new engine batteries?

 

I appreciate the idea of using a good scan tool while the car is operating as it should, and then again if (when?) the car is not acting as it should. I am just not excited about spending a lot of $ on a decent scan tool. Instead, using the on-board computer, I will periodically keep a record of the engine battery voltage when the car is off (no load), when the headlights are on (with a load), and when the car is running. If the readings change over time, then I will consider buying a scan tool to diagnose the issue after-the-fact.

Yes, as far as I know the brake light switch is reliably working. When my wife drives our Prius away from our house and she applies the brakes at her "first turn", the brake lights always illuminate. Why do you ask?