OEM S13 / S14 240SX detonation (knock) sensor.  Fits all KA24DE Dual Cam 240SX models. 

This sensor is a "microphone" through which the ECU listens for pre-detonation or spark knock that can cause engine damage.  In the event that the ECU "hears" detonation it pulls timing and adds fuel until the detonation goes away but it does not generate a diagnostic trouble code.  Unfortunately the S-chassis ECU does not "listen" for detonation at higher RPM, as it is not able to distinguish detonation from other engine noises. 

A diagnostic trouble code 34 generated by the ECU means that the circuit for the detonation sensor is broken, not that the ECU has "heard" detonation.  The ECU is always looking for a resistance in the detonation sensor circuit of around 1 mega-ohm.  If this resistance is not present the ECU throws a code 34 and also goes into a self protection mode in which ECU switches to more conservative safety maps with less timing and more fuel.

Normally the detonation sensors do not fail unless they are broken or melted as a result of overheating the engine.  The plastic casing begins to liquefy around 250 degrees F.  So this is a helpful indicator if an engine has been overheated or not. 

We do not recommend the cheap detonation sensors found on E-bay local auto parts stores and the like.  These cheap aftermarket reproductions seem to be incapable of picking up the proper frequencies required to identify detonation.