I have 1998 Nissan Frontier with an engine light on. I paid a mechanic to put in a speed sensor and the light came back on the day after he put in the new part. The codes before he worked on the truck were po500 and po325 and the codes are still coming up po500 and po325.He told me he did not replace the knock sensor. I don't know what to do next. I paid him and passed emmissions but the engine light came right back on.