Theory Of Operation

The Particulate Matter (PM) Sensor is used to determine the efficiency of the Diesel Particulate Filter (DPF) by measuring the amount of particulate matter (soot) remaining in the exhaust gas after the DPF. The PM Sensor is located at the outlet of the Selective Catalytic Reduction (SCR) Catalyst, and consists of a probe and module which cannot be replaced separately of each other and must be replaced as a unit. The module communicates and shares information with the Powertrain Control Module (PCM) over the J1939 Data Link. The probe contains three internal parts: a heater, a temperature sensor, and a set of particulate measuring electrodes with a high electrical resistance. During normal operation, the module will activate the heater and perform a regeneration of the electrodes to burn off any deposited soot before taking a measurement. After sensor regeneration is complete, soot particles from the exhaust gas are then collected onto the electrodes and form conductive paths between the electrodes leading to a drop of the electrical resistance. The decreased resistance results in an increased current across the electrodes, which is compared against calibrated thresholds to determine sensor functionality and DPF efficiency.

The PM Sensor probe must have a clear path to the exhaust gas in order to obtain an accurate measurement. Before starting a new measurement, the PM Sensor will perform a regeneration of the electrodes to burn off the deposited soot so that each measurement starts at similar conditions. The PM Sensor performs diagnostic checks to determine if the initial reading (at the beginning of regen) is inappropriately low by monitoring the heater voltage during the protective heating mode. During this protective heating mode, the speed of the exhaust gas moving past the probe will result in an increase in the heater voltage to maintain the required probe temperature. The heater's voltage change will be minimal if the sensor probe's protection tube is plugged, or if is the sensor probe is improperly installed into the aftertreatment. The regeneration is considered successful if the probe's heater is able to maintain the regeneration temperature and the PM Sensor transitions into measurement mode.