The temperature that is too hot for searing is said to be any temperature above 350 F but many people sear at higher temperatures and do okay.
Although a pan temperature of 400 F to 500 F is often recommended for searing, exceeding temperatures above 500 F can lead to burning of the oil and a bitter flavor in the food.
Temperatures above 350 F can slow down the Maillard reaction, that is responsible for browning and flavor development.
When searing it's best to heat the pan up first to 325 F to 375 F and then add the oil and get the pan to around 400 F to 450 F.
If you get the pan temperature too high when searing, you run the risk of burning the outside of the food, well before you cook the interior of the food.
Preheat a heavy skillet or pan over medium-high heat when searing.
Using high heat is important when searing, which will help the meat develop a rich, flavorful caramelization.
If you can't get a good sear on your steak then the pan is likely not hot enough.
A hot cast iron skillet will instantly begin searing steak.
Medium-high heat will cook the steak but it isn't hot enough for searing.
The second reason for not getting a good sear on your steak is likely that the steak is sticking or you are flipping the steak too soon.