Dash Warning Lights. A real headscratcher!

If the lights are on, then there are codes set. So if you're really using Techstream then that's messed up and I would hook back in and try again. I'd assume that's where you got the readings on the HV battery?

In any case, before anything else I would check the 12V aux battery, including load testing. When those get to teetering on the edge of death all sorts of weird things go on.

 

I agree with Josey that it is super unusual for Techstream not to be seeing the codes. Ordinarily, Techstream is what we suggest to people when they are using something else that isn't showing them the codes.

It could mean there is wiring damage affecting the diagnostic port, so that Techstream is not able to retrieve the codes.

Because the ABS light is on, you can at least get those codes, using the jumper wire, count light blinks method. That won't give you the codes behind the warning triangle, but some codes will tell us more than no codes.

If there's a PS warning, the Gen 1 steering ECU has a funny situation. It knows how to blink codes out on a dash light, and the wire harness even has wiring all the way from the ECU to the combo meter for a PS light, but there is no light there. When I had a Gen 1, I added an LED there (just under the right corner by the other warning lights) so I'd be able to see steering blink codes.

 

So that indicates there is a problem, possibly with networking, that needs to be solved first, so that the codes can be retrieved. There are troubleshooting steps in the repair manual for that.

In the meantime, codes from the brake system can be retrieved using the jumper wire, count light blinks method, which might not be affected by whatever comm issue is preventing Techstream from reading the codes. Steering codes can be read that way too, but only if you provide the light to blink, as discussed above.

 

Yes, I would replace that 12V before doing anything else.

 

That doesn't qualify as a dead battery without further investigation. If it can hold 11.4 V then it likely will just need charging. If it will charge up to 12.9 V and hold at least 12.6 - 12.9 V overnight, or hold 11.8 with a headlight load then you can call the good and move on.

There are so many stories of people changing batteries on this type of advice only to report that it made no difference to their symptoms.

While it is important to have a good 12 V power supply, automatically defaulting to replace with a new one is not very often the solution.

 

a) 12.2V w no load is basically dead. 11.4V w/ load is marginal to inadequate.

b) the 12V is old.

But ok. Put the 12V on a good charger. Resting it should sit at something in the range of 12.7-13V.

 

If by dead you mean flat, I agree. The fix for a flat battery is to charge it up. 11.4 V might be marginal to inadequate as a static voltage, but if it holds 11.4 V with a load, then it does not mean automatic replacement. The trouble with using just one factor is you cannot make a sound assessment. There are a several factors and these need to be looked at as group. It is in assessing all the relevant data that you can then you make a good assessment of the problem and then decide what is the best solution.

 

Sure, "flat" is fine rather than "dead" if you prefer. But a car battery - of completely unknown age - that is sitting at 12.2V either indicates that the battery won't hold a full charge or that the car has been sitting un-started for a really long time. (Or that there is a parasitic draw).

I know that reference to the 12V battery is a knee-jerk response to all sorts of Prius issues. Mine wasn't knee-jerk. Like I said, it's apparently old and 12.2V is ... well, bad. I've "been-there/done that" with unreliable 12Vs in my own Gen 1. I know first hand that you are very likely to get unreliable data (like error codes) and thus can't properly assess a problem if the 12V is marginal.

But, sure. Rather than just run out and replace it, charge it up fully with a good quality charger and then ... watch it vigilantly to see what it is doing. And if it's sitting at 12.2-ishV and dropping to low 11's when you power it up - don't assume the error codes aren't themselves errors. The error code thing is not as fool proof as some believe. Computers are relatively dumb.

I know that "opinions" vary about that. But like I said - I have been there/done that with problematic 12Vs.