Method for evaluating degradation of a particulate matter sensor after an engine start

An engine method, comprising:

• when starting an engine at an engine temperature less than a threshold temperature
• indicating a plugged sensor shield of a particulate matter sensor having two electrodes separated by a dielectric gap when a rate-of-change in conductance of an output of said sensor after said start is less than a threshold amount, said rate-of-change in conductance related to water vapor in exhaust gases generated by said engine during said engine starting.

A method for detecting a degraded particulate matter sensor, comprising:

• starting an engine with a temperature less than a dewpoint temperature;
• and
• comparing an output of a first particulate matter sensor to an output of a second particulate matter sensor, said output of said first particulate matter sensor and said output of said second particulate matter sensor taking place between a first exhaust gas temperature and a second exhaust gas temperature, wherein said first particulate matter sensor and said second particulate matter sensor are located in a common branch of an exhaust system of said engine.