so: can a Techstream system even access DTC's... if car wont power-on??

Probably won't work, but it is easy to try:

Maybe unhook the negative cable at the battery post, wait a couple of minutes, reattach and try again? This "fix" works when a logic board is stuck in a funky logic state, in this instance, one that prevents the car from powering up so that it cannot get out of that state. The longish power off allows time for the charge to drain out of the chips.

Again, probably won't work, but easy to try:

In the same "treat it like a computer" mode, hold down the start button for a long time (once the 12V is reattached.) The guess in this instance is that some logic board thinks it is on, and holding in power will generally force a shutdown. After that it might start with another press of START.

Yet another unlikely scenario that is very easy to test:

If the brake pedal sensor was broken the car wouldn't start with one START press. However, it should light up the dashboard. Unless the dash display has failed (the bad capacitor issue). Let's say that is the case. At this point the ODB2 should be alive even if the car isn't showing many signs of life on the dashboard.

My money is still on a problem with the START button: the switch is broken, or a wire has a problem, or a cable isn't plugged in. That sort of thing.

 

I just read swing's other thread, which preceded this one. It said that before the car was off for a very long time the combination meter was already acting up. The failing cap will have continued to degrade just from age while the car was off all that time. It may now be to the point that that display will not light up. The car should still start though, although it might not turn off without holding down the start button for several seconds. One point being, he should not expect with high probability that the dash will light up normally even when the START button issue is resolved.

Was the dash ever taken apart to try to remedy that combination meter issue? There are lots of connections back there which can end up not reconnected properly on reassembly, including many involved in the function of the START button. (When a dealer changed my combination meter they forgot to hook up one of the air handler cables, so there was no way to redirect air to the windshield or the floor. Not just any dealer mind you, but the biggest one in L.A., with an absolutely enormous service area.) Presumably if this is the problem it is not that there was no reconnect, but a loose one, and it fell out the rest of the way at some point.

 

Responding to the communication question

Yes, i am able to pull codes from a completely shut off as in power button does nothing prius via techstream and an amazon cable, along with my dad's expensive af autel.

You can only communicate with 5 ecus, if those ecus are getting power(like they should)

I recently had inverter pump failure and with am2 blown i could only communicate with like 1 giving a combo meter code. Replaced am2 fuse, prius came back to life. Limped to buy overpriced autozone pump while watching inverter temps with a elm1327 scanner. Replaced pump in under 10 minutes. Secret is to pull the drivers side headlight. 4 bolts total. 1 on bumper, peel bumper back, 3 on headlight. Huge amount of space to work.

Now i gotta replace the autozone pump with an aisin, amazon sells the oem pump for around $110 but i couldn't wait, was in 105f palm desert, ca area working a music festival. Metallica was epic.

You should be able to communicate with (iirc)

Gateway, body controller, smart key(certification ecu), immobilizer, and one more

If you check my posts/threads about my dead prius issue, i detail the information there with proof of concept using an autel(model is listed in my thread, its $$$ like 4-5k). Amazing a $40ish amazon cable and pirate techstream can do the same.

More stuff that may be of use

https://forum.ih8mud.com/threads/how-to-techstream-in-5-minutes.1034923/

This is a super easy method of installing techstream, if i recall it's version 12. Works on windows 7,10,11, even mac os. Using virtual images. All files required but you may have to download some Microsoft run time/java stuff that is easily found on google. Saved my butt before with my corroded wire harness issue, and saved my butt recently 500 miles from home with inverter pump failure. Which smelled terrible, burning wire smell for a couple days. Thought my actual inverter died.

I purchased this cable for techstream couple months back.

Aidixun Mini-VCI J2534 Cable Firmware 1.4.1 for Toyota Techstream, Toyota Tis Cable,Techstream Software (Newest Version) https://a.co/d/iAQ2xPK

They're cheaper ones but this had same day delivery. Works on my 2010 camry and both priuses. Pulls codes with prius completely powered off via techstream linked above.

Hope this helps others!

 

Only buy Toyota OEM parts from a dealer. Amazon is awash with fake parts, many in more or less authentic looking packaging.

 

Let us know what they find…

 

I instead purchased it from rockauto. Aisin store on amazon is selling the pump but would rather not have a headache.

Thanks! Pump is only $92 on rockauto.