real bad head gasket or stuck thermostat You might want to check the oil and see if it is a milky white. Also look at the exhaust to see if it is blowing white smoke. It could possibly mean blown head gasket, or a cracked head.
yes
Check the engine coolant... Is it full? yes coolant level is full
The engine is overheating. The check engine light is not on because you have no problem with the emissions system which this light monitors.
It is possible that a check engine light on a Toyota Highlander will come on if the coolant level is low. Usually, though, there is another light that will come on that says 'low coolant' on some older Toyota's.
Low coolant would be because the coolant is low or the sensor has failed, Check engine light would be because the computer has detected a problem and set a code.
start with engine off. press and hold that speedometer resetter while starting engine, check engine light will flash a few times then disappear.
when the engine is cool, check the anti-freeze level. it may be low.
Disconnect the negative battery cable for a couple of minutes, then reconnect. The CHECK ENGINE light should disappear if everything is okay.
Yes, on some vehicles it can.
Check to coolant level in the coolant recovery tank and radiator when engine is cold
Your check engine light will not come on because your coolant needs to be changed. The only way to be totally sure why it is on you should get it checked with a computer at a dealer or service station. The check engine light may come on if it is over heating because there is no coolant in it but not if it just has to be changed.
It could be that your coolant level is low or you have other coolant system problems. Whenever the check engine light is on, use a code scanner to find out why.