Main relays do tend to die in the heat, but they don't seem to care whether the engine is warmed up or not. They're just as likely to die on the first start on a hot day -- even though the engine is "cold." Main relays usually die because of a solder breakdown on their board, and this is affected by ambient air temperature inside the car, not engine temperature.

But here's how to be sure: If you hear the fuel pump kick in when you turn the key to ON, the main relay is okay. If you DON'T hear the fuel pump kick in, then the main relay is PROBABLY bad.

Sometimes, though, the TW sensor (coolant temp sensor) dies, and they frequently die in a funny way on Hondas -- they die "cold."

What I mean is, they track the coolant temperature from cold to hot and they tell the ECU what's going on. I think they put out .5V for a cold engine. As the coolant temp rises, so does the voltage, until it hits around 4.5V, maybe 5V for a fully warmed-up engine. Not exactly sure of the voltages here. But then, for some strange reason, they "go cold," putting out around .5V again, even though the engine is really fully warmed up.

In normal driving, you won't notice the difference. But when you try to RE-START the car, the ECU sees START and COLD, and so it dumps all kinda fuel into the injectors (which it SHOULD, to start a cold engine). This'll flood out a hot engine, though, and so it won't start.

Once the engine (and TW sensor) cools off, everything's fine again. Until the sensor warms up and "goes cold." Then, the cycle starts all over again. You can watch it on a voltmeter.