My Blobeye WRX (EJ205) has been struggling to get up to normal working temperature recently. The temperature gauge will sit at the bottom for ages before creeping up to at 1/4. It might reach 1/3 when sitting in traffic. I've burped the coolant a couple of times, so seems to be free of air pockets. The heater seems to be working OK, so the coolant is getting through the heater matrix.
So could the thermostat be stuck open?

 

My WRX wagon has had an aftermarket aluminium radiator fitted which I think is larger than standard.
it also takes a lot longer to get up to temperature than what I remember the standard one did.
I'm pretty sure the gauge doesn't go as high either.
What rad is on yours

 

Standard radiators are more efficient than the aftermarket bling radiators. The only reason they take longer to warm up is cause they hold twice as much water which isn't a good thing either.

 

It's running a standard radiator, which I installed a couple of years ago.
I plugged in my ODB-II tool and it read the coolant temperature as around 185-190 degs F (so 85-88 deg C in civilised units) after a decent run. So not sure if that's within the normal range?

 

Blobeye thermostat info

 

As my other thread about my coolant leak, it seems that my temperature gauge is under-reading. When the engine overheated it barely got to halfway. At least this particular mystery is solved.