here is a quick how to on how to diagnose a bad knock sensor. last night it was a WTF am i missing, but apparently i have 3 bad knock sensors.

ok so you have code 5, the shop manual Foxed.ca - Mazda RX-7 Manuals, basically just says to check continuity and if its good then replace with a known good sensor. the trouble i ran into was that i had a sensor that was fine, i replaced the harness, and then it wasn't fine. i tried 2 more sensors, and a second ecu so it was time to go to the next step.

this is to hook an volt/ohm meter to the knock sensor wire, and then pin 3M on the ecu, they should have continuity. or you could turn the key on engine off, and check voltage at the k/s connector, it should be around 5v. if you have both of these, the wiring is good.

testing the sensor is harder, as there is no spec on them, but Speed of Light, has found that you can put a 270K ohm resistor between the k/s connector and ground and the ecu will see it and turn off the CEL. i confirmed this with mine, it does work.

i couldn't get any kind of reading out of any of the 3 sensors i have, so knowing about the resistor makes this really quick and easy. i used a 250k ohm potentiometer out of a guitar, although

hope that helps!

 

oh just a follow up, the 4th sensor had resistance, and the ECU is happy.