Pontiac Sunbird 2.0 4cyl. Code 15

I have a Pontiac Sunbird 2.0 4 cyl with 132K.

While running for approx. 2 minutes, the cooling fans both come on, along with the check engine light.

I pulled codes, and found a "code 15" Low Coolent Temp

I've replaced both coolent temp sensors ( The one in the head, and the one in the thermostat housing ).

I've also replaced the thermostat, and the ECU, too.

What am I missing, here?

Thanks much,

S.