Ok, got the thermostat replaced, it's not that bad, the code that it was throwing was P0128.

Replacing the thermostat cleared the code, this car, at a grand old age of 316.500 miles on it now has zero codes!

I'm glad I did it, the rubber gasket on the old thermostat was split. fraying and a couple of small bits of rubber in the housing.

This is the thermostat for a 2003 Civic Hybrid - Honda Thermostat Assembly OEM 19301PZA305, only recommend using OEM, as the hybrid engine is a little different to regular Honda engine.

And one of these is so useful, I will never do coolant drain refill without one of these now, it made bleeding idiot proof - Thorstone Spill Free Radiator Coolant Filling Funnel Kit https://smile.amazon.com/stores/Thor...C?ref_=ast_bln

Coolant, get it from Honda, but I used this Zerex 1 Gallon Blue 50/50 Coolant/Antifreeze #861398

Car must be flat and level
Warm car up with the heater on, then turn off. Let cool down for 10mins or so.
Drain coolant, unscrew petcock on the underside of the radiator, CAREFULLY (it may still be very hot) unscrew the radiator cap to speed of coolant drain
Remove battery and battery box
Remove air intake, air filter housing assembly
Follow the radiator hoses up to the right side top the engine where they connect to the thermostat housing
Loosen remove the hose clamps on the hoses that connect to the thermostat, on one of them it has a metal connector hose/pipe, I removed the hose from the metal hose, and then wiggle the metal hose/pipe out of the thermostat (it has an O ring)
Remove the small upper hose
I left the 2 hoses going to the heater core attached, they look difficult to remove, you can easily work on it with the hoses still attached to the heater core and the thermostat housing.
I then place a big screwdriver and popped the thermostat cover off
There is no gasket on the thermostat side, the thermostat has its own rubber gasket, DO NOT use gasket maker on this side
Install new thermostat, it is keyed, it should lock in
On the other side though you will have to make a new gasket, as there is no ready made gasket from autoparts stores, I used grey Permatex, https://www.oreillyauto.com/detail/b...ufacturer=true.
Put it all back together.
Then use the Radiator Coolant Filling Funnel Kit to refill it.
TIP - there is a bleed nipple very close to the thermostat housing, 12mm, use this to help with the bleeding.
Spend time bleeding, it will take around 30 mins to bleed properly.