The oxygen sensor is sealed against moisture and contamination inside a solid shell-like casing. The sensor has a zirconium dioxide sensing element, heater contact, heater element and cable connector with wires that lead to the ECM. Some vehicles can have multiple oxygen sensors - as many as four– located in front of the catalytic converter and some placed behind it in the exhaust pipe. The vibration signals can be inaudible to the human ear, making knock sensors highly sensitive.Īn oxygen sensor is a small tube-like component that screws into the exhaust manifold or header pipe in front of the catalytic converter.

The ECM reacts by decreasing the ignition timing in small increments, usually two degrees at a time, until the vibrations or knocks are decreased and eliminated. The small microphone-type sensing element inside the knock sensor reads any abnormal fuel-burning characteristic through vibration, and sends an electric signal to the ECM. Pre-ignition happens if the fuel has the wrong octane level, incorrect timing, uneven fuel burning and, in some cases, can be caused by hot spots inside the cylinder. Knocking or pinging during engine operation is the result of pre-ignition. This information is relayed to the ECM through wires connected to the sensor. A knock sensor incorporates a small piezo crystal and shunt resistor inside its case, designed to pick up minute or heavy knock vibrations in the cylinder or cylinders. Basic differences exist between the function of knock and oxygen sensors.Ī knock sensor is a small cap or plug-like component that usually has a threaded end that screws into the engine block near the cylinder or valve locations, and often is placed at the rear of the engine. Both devices relay information to the ECM, or engine control module, to adjust fuel delivery to the cylinders or modify the ignition timing.

Oxygen sensors, which are mounted in the exhaust system, detect unburned oxygen levels in the emissions gases after combustion.

Knock sensors monitor the combustion cycle of the engine, determining if the fuel is burning cleanly and efficiently, or burning too soon, by 'listening' or detecting audible knock vibrations during cylinder firing. Knock sensors and oxygen sensors, or O2 sensors, function as sensing devices that detect variations in an automotive engine’s performance.

Knock sensors function