Possible Causes

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

Possible causes are:

No knock sensor signal to the ECM with engine speed, 2000 RPM or more.
Knock sensor circuit is open or shorted.
Defective or loose knock sensor.
Defective ECM.

If the ECM detects the above diagnosis conditions, it operates the fail safe function in which the corrective retard angle value is set to the maximum value.