A method for monitoring a particulate filter arranged in an exhaust line of an internal combustion engine, said method comprising the steps of: monitoring the exhaust gas stream downstream of the particulate filter by means of a downstream soot sensor, said downstream soot sensor having a characteristic sensor cycle during which particulate matter accumulates up to a predetermined threshold; monitoring the accumulation of soot at an upstream soot sensor over a respective sensor cycle of the downstream soot sensor; deciding on the particulate filter operating status based on the information of the downstream and upstream soot sensors; wherein the upstream soot sensor has a characteristic sensor cycle during which particulate matter accumulates up to a predetermined threshold; and the amount of accumulated soot is determined based on the sensor cycles of the upstream soot sensor, and expressed either as a soot loading level or as a number or frequency of sensor cycles, wherein said upstream soot sensor is provided by a virtual sensor model having as input a number of predetermined engine operating parameters, said model comprises a transfer function outputting a model sensor response time, preferably corresponding to the response time in steady state corresponding to the current engine operating parameters, wherein a soot indicator representative of the soot loading level of the upstream soot sensor is iteratively computed based on a portion of model sensor cycle time corresponding to each iteration period over one sensor cycle of the downstream soot sensor; characterised in wherein the following routine is periodically performed: determining a status of the downstream soot sensor and the soot indicator at the current timing; if the downstream soot sensor has not finished its sensor cycle: comparing the soot indicator to a pass threshold, and if exceeded, comparing an average of prior soot indicator values to the pass threshold, and, if exceed, the particulate filter is considered to have a functional operating status; or if the downstream soot sensor has finished its sensor cycle and the soot indicator is below a fail threshold: an average of prior soot indicator values is compared to the fail threshold, and, if below, the particulate filter is considered to have a faulty operating status. monitoring the exhaust gas stream downstream of the particulate filter by means of a downstream soot sensor, said downstream soot sensor having a characteristic sensor cycle during which particulate matter accumulates up to a predetermined threshold; monitoring the accumulation of soot at an upstream soot sensor over a respective sensor cycle of the downstream soot sensor; deciding on the particulate filter operating status based on the information of the downstream and upstream soot sensors; wherein the upstream soot sensor has a characteristic sensor cycle during which particulate matter accumulates up to a predetermined threshold; and the amount of accumulated soot is determined based on the sensor cycles of the upstream soot sensor, and expressed either as a soot loading level or as a number or frequency of sensor cycles, wherein said upstream soot sensor is provided by a virtual sensor model having as input a number of predetermined engine operating parameters, said model comprises a transfer function outputting a model sensor response time, preferably corresponding to the response time in steady state corresponding to the current engine operating parameters, wherein a soot indicator representative of the soot loading level of the upstream soot sensor is iteratively computed based on a portion of model sensor cycle time corresponding to each iteration period over one sensor cycle of the downstream soot sensor; characterised in wherein the following routine is periodically performed: determining a status of the downstream soot sensor and the soot indicator at the current timing; if the downstream soot sensor has not finished its sensor cycle: comparing the soot indicator to a pass threshold, and if exceeded, comparing an average of prior soot indicator values to the pass threshold, and, if exceed, the particulate filter is considered to have a functional operating status; or if the downstream soot sensor has finished its sensor cycle and the soot indicator is below a fail threshold: an average of prior soot indicator values is compared to the fail threshold, and, if below, the particulate filter is considered to have a faulty operating status. determining a status of the downstream soot sensor and the soot indicator at the current timing; if the downstream soot sensor has not finished its sensor cycle: comparing the soot indicator to a pass threshold, and if exceeded, comparing an average of prior soot indicator values to the pass threshold, and, if exceed, the particulate filter is considered to have a functional operating status; or comparing the soot indicator to a pass threshold, and if exceeded, comparing an average of prior soot indicator values to the pass threshold, and, if exceed, the particulate filter is considered to have a functional operating status; or if the downstream soot sensor has finished its sensor cycle and the soot indicator is below a fail threshold: an average of prior soot indicator values is compared to the fail threshold, and, if below, the particulate filter is considered to have a faulty operating status. an average of prior soot indicator values is compared to the fail threshold, and, if below, the particulate filter is considered to have a faulty operating status.