Coolant Boiling in Maintenance Mode

My 2012 has been going through a lot of self maintenance lately but this recent one has stumped me.

I got an overheat warning last night. Oil & coolant levels are good. I relatively recently replaced some items in the coolant system and heard that rushing water sound behind the wheel last night, so I thought there might be some air pockets. This morning, I put it in maintenance mode, raised the coolant tank, and let it run with AC and cap off. I was seeing bubbles come up, but about 15+ minutes into it running, the coolant tank started overfilling. Fans weren't running with AC off, so I turned on the AC to make sure the radiator fans worked, and they did. Not losing coolant, no white smoke - so pretty sure it's not head gasket.

Relatively newly replaced items: water pump (2.5 months), radiator (2 months), EGR cooler (1 week), and air filter & MAF sensor (3 days).

What have I missed that could be causing this boil/overheat?

 

~227k miles - I've had it about 5 years. Got it used at around 112k.

 

Thank you for the list - few Q's
1) Does that mean there are 2 different ECT sensors?
2) Not exactly comprehending the scanner concept, could you expound on that point?

 

Yeah, the scanner thing will probably have to wait until I can afford it. Uber is my job and this kinda halted my work till I fix this.
Ok, so I assume the 2 ECT sensors is the one just under the EGR cooler, and the one under the wipers where the bleeder used to be pre-2012? Which one is 7C4 vs 7C0? Is there a usual suspect as far as replacing?

If it matters, I got the AISIN WPT19 water pump from Rock Auto, so chances seem low of it being bad, I'd think?

 

Long-term, you're absolutely right. Unfortunately, I'm in a "right-now" barely making it situation. Unfortunately, this particular overheat doesnt throw codes, just the overheat indicator. Thankfully, my friend had scanner with some basic datastream functions (iCarsoft MBII) so I'm looking at that now.

I've replaced the ECT sensor on the engine side and cleaned the sensor near the non-bleeder (it had some kind of gunk on it). So far, idling on maintenance mode hasn't made a fuss with the coolant, and the reader didn't go over 96°C, even when revving. Reservoir didn't fill up at all.

Took it for a drive in-neighborhood (<30mph) with AC running. Temp got as high as 110°C near the end of the 10 minute drive. Heard some water noises behind the wheel. Will need further testing.

Now that I have a way of reading the coolant temp, what is an OK range? I was reading operating twmp should be 90-95.
Do y'all know at what temp it throws the warning light? (Google failed me when trying to find that)