Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I am having an overheating problem only at idle.The cooling fan does turn on and it seems like it’s running fine but I think the higher speed turns on too late. I have noticed that if I give it gas and drive around it will show normal operating temperature pretty quickly. Thermostat seems fine. Head gasket chemical test seems fine. Water pump doesn’t appear to be leaking. I have a very tiny leak from what looks like a grommet on what I think is the automatic transmission cooling line. But coolant level doesn’t seem to be effected over the course of a couple weeks.
Something else I have noticed is that I have an 02 problem or a plugged cat. One bank looks fine. But B1S1 looks functional but B1S2 shows .000 unless given gas. I have basic diagnostics tools to check electrical and see live data but no back pressure tester and no radiator pressure tester.
What engine is it?
Does the cooling system have low pressure tank?
Is the fan blowing hot air when is active?
Have you check without thermostat?
Have you checked coolant flow?
