1996 nissan sentra OBD P0500 and P0400

Help! I feel like a lamb being led to slaughter. In trying to get my car passed its annual smog test, these two codes failures appeared. My mechanic says only the dealer can make the repairs. The dealer refuses to provide even a ballpark estimate of what the repairs might cost or even if the repairs will get the car past the smog test. The car seems to run fine. So far, I've determined that P0500 may have to do with the speed sensor. My speedometer seems to work just fine. And, although the check engine light is on, the car seems to run fine. It passed the emissions test, by the way. Any help will be greatly appreciated.

not much of a mecahnic if he thinks only the delaer can fix those 2 codes, a speed sensor circuit code and an egr valve code, ooh tough!

As with many Nissans that I have worked on the PO500 code could be caused by a problem with the speedometer head itself. You mentioned that the speedometer works fine but I have seen this problem in the past and on the cars that I fixed the speed sensor code was present and the speedo also worked normal.

What I have found was that the signal from the speedometer head to the ECM/computer was bad. There was no output signal from the speedo head itself. I have replaced the speedometer head on several Nissan models to fix the problem.

You could try a used part but if the odometer is part of the speedo head the mileage may not match what you need. I prefer to order a new part from a Nissan dealer. If your sentra has the odometer built in you will need to provide the dealer with your VIN#/ vehicle ID number and the current reading on your odometer so they can order the part with the correct mileage on the new odometer. The dealer charges a fee to adjust the mileage on the new part and I'm not sure if they will sell it without the mileage adjustment.


The PO400 code could be any one or more possible things which are related to the ERG system. One of the problems I see on many of the Nissans I've serviced is a faulty EGR solenoid valve. You will need to have the EGR system serviced to find the exact problem since there are many other possibilities.

Good Luck!!!