I have a 2003 Highlander. Whenever I get the oil changed, the check engine light comes on within a couple of days. I finally figured out that there is a hose under the air filter case that gets disconnected when the filter is checked. Just reattach the hose and the light will reset after a few restarts of the engine.
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.
Check your brake (stop) light fuse
Have vehicle scanned to determine the problem - usually in the emission control system,
Have vehicle scanned to determine the problem - usually in the emission control system,
start with engine off. press and hold that speedometer resetter while starting engine, check engine light will flash a few times then disappear.
Low fluid levels in engine.
Problem with the emissions system.
The ECU has detected a problem with the emissions system. Have the ECU scanned with an OBD2 scan tool to retrieve the code that has been set. This code will tell you where to start looking for the problem.
Go call the Toyota car company or headquarters and ask the people who made the car.
To reset a check engine light for a Highlander 2003, disconnect the battery for 10 minutes. After this time, reconnect the cables.
For the Highlander 4 cylinders, it is on the Alternator side. To replace it, you have to take the alternator out and unscrew two bolds . Unless the engine is rebuilt, thermostat only replaced, most likely, when you rebuild the engine. check cooling tank and water hose leak first.
It could be anything, but the most common problem is the O2 sensor.