2005 Civic LX at 210k miles, just started to overheat tonight. The first time this happened I blasted the heater to lower the gauge before turning it off, but this time there was no heat at all at idle/moving. The first time it ended up being the fan switch, I'm not sure this time. Based on previous threads I hope it isn't the head gasket. I replaced the water pump at 180k in January 2019. When I checked under the hood after it got hot I heard some hissing, and noticed a leak under the block, but this might be oil from another issue. Could it be the radiator? The top hose from the thermostat felt hot and the bottom one was cooler but still warm I don't think that's the issue since there is no heat in the cabin. It's odd compared to last time because it just started and it overheats almost immediately after reaching operating temp. Thanks for any help.

Photos: https://imgur.com/a/M1PY6xK

 

I replaced the thermostat today to start with the simple stuff it might be and the heat is working again while driving/idle and it hasn't overheated. However I noticed some bubbles in my power steering fluid while burping the coolant, as well as the vacuum reading fluctuating a lot while driving on my OBD2 reader, it was reading negative values and dropping to almost 0 and while accelerating even moderately. I've noticed the car has been lagging a bit when I press the gas pedal and I've been abusing it a bit by only driving short trips.

 

Lack of power can be a head gasket problem , with that many miles and an over heating experience maybe a compression test is in order . And I wouldn't put a lot of faith in the thermostat if it was a $10** auto zoo one .