"I don't have any experience with on-board diagnostics other than a few times when the MIL light has come on in my own car, and then finding out later what the diagnosis at the dealer service center was. However, I am aware that the position that I am interviewing for is for an entry level engineer to work on diagnostics. So I have read a bit about it in preparation for our chat today. I know that the purpose of on-board diagnostics is for the engine management system to constantly monitor whether all the elements of the emissions system are working properly. I know that the diagnostics check for circuit faults like voltage high or low, or open circuit. I also know that there are classes of faults detected by rationality diagnostics, where the software can assess whether the device is giving a result that doesn't make sense for the current operating conditions.
In reading about it a little bit, I learned that if the criteria for passing a diagnostic are not met for a certain length of time, a malfunction code is set, and that with a scan tool, one can read the malfunction code and have a pretty good idea where to start in troubleshooting potentially bad hardware. I imagine that one of the biggest issues in designing diagnostics would be avoiding false failures and false negatives. It sounds like a really challenging field of work."
