What triggers code 31 (knock sensor)?

[Diz]

What is the logic that triggers this code? Zero signal from the sensor? Excessive voltage? Why would it trigger with the sensor disconnected? I'm assuming the sensor generates a small voltage signal during knock and none under normal conditions...

 

[Agent Ezzard]

In OBD2, too much and I mean a lot of know would cause the ECU to throw a hissy fit. So excessive voltage is correct.

 

[1gawddsm]

if you have the knock sensor unplugged you should get a default value of 9 counts of knock while driving. if the knock sensor in plugged in, but not screwed into the block i believe you get the code you are talking about. the knock sensor is electrically grounded though its physical contact with the block when it is screwed in.