2013 Prius Help (B1503/Check Hybrid System)

all!

TIA for any insight you can provide.

2013 Prius Two, 138XXX miles, bought used just over a year ago. Previous owners kept up with routine maintenance. She's mildly overdue for an oil change.

I had the dreaded brake actuator assembly replacement recently and am looking to avoid returning to the dealership with what I'm encountering now. One $3.5k bill was more than enough lol. Apologies if I overshare unimportant information or seem all over the place, however, I've been scouring this forum, YouTube and the Prius subreddit to try and find an answer for days and can't figure it out.

FWIW, I'm in the Tampa area, so there aren't any elevation changes but the weather's been hotter than it has been the last few years.

Prius stays garaged unless being used. Factory tint from what I can tell.

I can run the car up to an early morning appointment or to work and back (simple tasks with limited driving) with no issue. I find if I'm making multiple stops or sitting for a while, I run into this issue.

The temperature light will come on momentarily, then the check engine light, and then I get the "Check Hybrid System" with a yellow triangle. Normally, I can pop the hood and disconnect the heavy white plug from the fuse box, wait five minutes, and it resets just fine. Except that doesn't always work. (When researching fixed for this, I got 1. the inverter is bad or 2. the fan to the hybrid battery is blocked/dirty/etc.)

For instance, I ran a kid to an appointment yesterday, got smoothies, and stopped at one more place, and by the time we were close to home, it happened. (Car was out of the garage for about 3-4 hours, parked in the sun, active driving maybe half of that). I was able to limp it back into the garage and when I popped the hood, it felt way hotter than normal. I disconnected that white plug and let it rest.

I ran Techstream in maintenance mode this morning and got a code of B1503 - Exhaust Heat Management Warning. There is no check engine light. I haven't driven it. Working through the Repair PDF on B1503 posted in another thread, I was able to determine the coolant temp is fine. The next step is to check the G2 Water Temperature Sensor and the L27 Combination Meter Assembly. I can't find a clear diagram of where those are to even check.

All coolant levels are full. There is movement in the coolant tank closest to the inverter so a block is unlikely.

Has anyone experienced any of this? Any assistance would be greatly appreciated!