It's hot enough to make you strat pulling the seat buttons out with your backside and praying that it doesn't let go.
When the light starts flashing it's best to do something about it.
Mine did it after Bramcote towing the caravan. Loads of water to wash the mud out of the radiator cured it.
I can't find any data with the precise temperature though. You could get technical and test/meter it in hot water until it brings the light on.
Ed