I have a 2002 Toyota Sequoia Limited that is overheating when it's running slow or idling. It starts to heat up slowly - not overheating right away. My mechanic is tell me it's a failing head gasket, but he didn't run any diagnostics. Any input?

 

It could be as simple as needing a new radiator cap. How many miles since the water pump/thermostat last changed? Coolant level ok? Any external leaks?

Do you have any of the classic head gasket failure symptoms: billowing white exhaust, boiling coolant escaping the radiator/overflow, oil in radiator, coolant in oil, bad idle, etc.?

Also, do a search here for testing the fan clutch.

 

Does kinda sound like the fan clutch if it's happening at slow speed.

 

Isn't it the case that all one had to do is crank car with hood open and see if the fan is running

 

I had same problem with 91 4Runner 3.0 I flushed the radiator,and installed a new thermostat,not that this was part of the issue I replaced the hoses as well being as they were a bit thin,and I had the coolant system already drained.I added a decent coolant,hint not the 50/50 type.Problem fixed.This is also a good time to check your belts for cracks,and replace as needed.Good luck out there.