Intermittent Red Triangle

First, let me say I am not a mechanic but last year I managed to diagnose and fix a 2001 Prius with the help of everyone on this message board. It had a voltage leak in the transmission. So thank you.

I am now having a problem with a 2002 Prius that I would like some input on.

The Red Triangle of Death appears only after cold starts. After the car is turned on, the code appears after about 30 seconds. It disappears after normal driving for about an hour (assuming the car is restarted several times to reset the code). It then never reappears until the car sits for at least several hours, but it usually needs a longer overnight rest before it comes back. It has been consistently behaving in this manner since May - it is never worse, but there was a period of time when the code did not come on even after a night's rest. After a few days of no driving it would reappear, even then. At no point has there ever been any problem actually driving the car. It behaves normally.

My scanner shows no codes at all. I borrowed the scanner from the local autozone, and it showed two things:

One P0302 code pending.

There was also freeze frame data that read simply P3009.

I was not happy to see anything about a 3009, but I thought maybe since it was only a freeze frame and not a code it was just left over from some repair job or something.

When I cleared the codes this erased both the P0302 and the P3009 freeze frame - but when I restarted the car the Triangle came back, and the scanner did not read any code (including the P0302) but the freeze frame P3009 returned. Therefore, I believe the Triangle represents P3009 and it just isn't showing up on the scanner as a code for some reason.

The next bit of diagnosis I did is the "park, neutral, drive" test to isolate the voltage leak. According to this test, if the P3009 is triggered when the car is turned on and in park, then the leak is in the battery. If it doesn't come on until the car is in Drive, then the problem is in the transmission.

In my other car, the 2001, the P3009 only came on when put in drive and the problem did in fact reside in the transmission.

In the current car, the 2002, the code returns even in park. This suggests the short is in the battery. I also have a scangauge which shows no abnormal temperatures in the transmission or odd readings anywhere else. In the 2001 with a transmission problem, one of the electric motors was running at a very high temperature.

All of this points to a battery problem, maybe just a short caused by corrosion. This does not seem terribly unexpected to me, as the car has 194,000 miles on the original battery. I am not against taking the battery out, clearing any corrosion out, and reassembling it to see if that fixes the problem. I have removed and reassembled batteries before. I even have extra battery cells so I could test each cell in the car and replace the weak ones, if I feel like spending a few days doing that.

That is a lot of work though, so I'm hoping to rule out anything else first.

My biggest question - if it is a battery problem, why does it go away when the car warms up? I would think a battery leak would be there at all times. If anyone can explain this behavior and maybe give me their two cents, I would appreciate it. Like I said, I think I've diagnosed the problem pretty well but I'd love to hear from the board before I do the battery disassembly.

Is it simply the battery expanding and contracting during use?

The only other thing I can think of to do is go to a dealer, but with the other car they consistently told me they wanted to replace the battery even though the transmission ended up being the problem, so I have no faith in them.

Thank you,
Brett

 

Is there any way to differentiate the ECU or am I just eyeballing the whole battery and deciding if the corrosion looks bad enough? I do have a spare battery ECU lying around.