I drive a 2005 civic coupe, it's been very reliable. I drove it from Cali to Colorado no issue whatsoever. I've replaced all engine mounts, radiator twice, new windshield, new ac condenser, new battery & cables, new starter, new ignition. All were preventive maintenance but last week or so my car starts to get up to operating temp quick & after 20 min or so it starts slowly rising to overheating, I've replaced the temp sensor, I've replaced thermostat & everything is in working order. There are no leaks, I lost my reservoir cap awhile back before my Colorado trip & no issue. I don't know why it's slowly overheating, any help would be greatly appreciated guys!

 

Breached head gasket is my guess based on the symptoms of overheating and coolant purged into the reservoir tank (missing cap). Plane the head before installing new head gasket. Common issue for these engines.

Overheating and misterious coolant loss common causes - reference - Honda Civic Forum (civicforums.com)

 

replace the thermostat and see what happens