Possible Causes

WARNING: This page is about a different car, the 2003 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Connector or terminal malfunction
ECT sensor malfunction
Open circuit between ECT sensor 2-pin harness connector terminal "A" (White/Blue wire) and PCM 104-pin harness connector terminal No. 38 (Light Green/Black wire)
Open circuit between ECT sensor harness connector terminal "B" (Brown/Green wire) and PCM 104-pin harness connector terminal No. 91 (Brown wire)
PCM malfunction
Short to power circuit between ECT sensor 2-pin harness connector terminal "A" (White/Blue wire) and PCM 104-pin harness connector terminal No. 38 (Light Green/Black wire)