Are these new or used sensors? They're VERY delicate devices and it's not unheard of to end up swapping out a handful of used sensors before you find a good one. I don't know if the thread is the same but in the MR-2 and Celica communities they've taken to using an adapter to swap in inexpensive GM knock sensors with the S series engines (5S and 3S, specifically). You might look into it and see if it'll work on the 7M.
Given that the knock sensor is a piezo electric device an abnormal knock or rattle may cause a signal to be sent to the ECU which is causing the ECU to think there's an abnormality in the circuit.
That's all I've got as far as suggestions.
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