Fans never come on!

My fans do work when hooked to the battery and i have checked the relays as far as I can tell they work too. It seems when I drive it it stays cool enough but when I drive slow or let it idle coolant boils out of the overflow. Why aren't the fans working when this happens? The car is a 98 M6 Z28.:giveup:

Air in the system?

Thermostat going bad?

The temp sending unit isnt kicking the fans on?

If the SES is on see if you have any ECT or fan circuit codes.

Quote:

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Originally Posted by CamaroFan71

The temp sending unit isnt kicking the fans on?

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Is that on the block/heads somewhere? I know the low coolant light is on. And the car is full. I doubt it has dex-cool in it either. It does throw ses but when I checked it I remember getting egr inop.

Quote:

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Originally Posted by kensz28

Is that on the block/heads somewhere? I know the low coolant light is on. And the car is full. I doubt it has dex-cool in it either. It does throw ses but when I checked it I remember getting egr inop.

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Its on the driver head, around the 1st plug.

Quote:

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Originally Posted by Y2KPewterSS

Its on the driver head, around the 1st plug.

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You're talking about the coolant temp sending unit? I know what that is, am I missing something else? My gauges seem to be accurate. What exactly triggers the fans? The ecm?