Started my 96 Honda Odyssey yesterday morning and went back inside for 10 minutes. When I came out, the the engine had overheated and there was antifreeze all over the ground.. Where should I start to diagnose the problem ? Thermostat ?

Did the cooling fan come on? if it did i would replace the thermostat,if it didnt i would fix that first,,

1st refill the radiator and overflow bottle. Start up the Honda. You should have a bleeder bolt on top of the engine. You need to open this bleeder to remove any air in the cooling system. Wait until the engine is warming up, open bleeder and when no air is evident, close it. There are a number of reasons it may have overheated, so carefully watch the temp gauge. If it starts to rise out of the normal range, check to see if the cooling fan is kicking on. If it isn't, you may have a faulty switch that should be signaling the fan to go on as the temp goes up (or possibly a defective fan motor). Check for leaks also. Have you noticed any prior to this problem? If the fan is operating as it should, air is out of the system, you could check the thermostat. Remove, place in a pot of boiling water. The t-stat should open rather quickly. If it is defective, it will remain closed, and could cause the overheating problem (it can also stick open, not allowing the car to reach operating temp, but that's not the problem).So, hope this helps figure it out, there are other things (radiator plugged up w/ rust and sediment), broken belt, so the water pump isn't spinning, etc Good luck, let me know what you find.

If the engine is an f22b or f22b6 the thermostat is towards the back left side of the engine. It is actually not too terribly difficult to get to. When you open the hood you should be able to see it just to the back/left of your intake manifold. It usually has a green plug in the top of it. and of course your radiator hose will hook to it also.