Check engine light still on after diagnostic code read and repair made

My 1996 Nissan XE pickup’s check engine light came on recently. I took it to my mechanic, who ran diagnostics on it and said I needed a speed sensor. He replaced it for about $350 parts and labor but the check engine light came right back on. So my mechanic ran another test, and once again the diagnostic code was for the speed sensor. Thinking the first part he used might be bad, he replaced the sensor once more with a new part, and once again, the check engine light came right back on. Does anyone have any idea what the problem might be? Unfortunately I can’t get a legal inspection sticker until this problem is solved, and my mechanic doesn’t seem to know what else to do. The truck is running fine and I take good care of it, so it is frustrating that I can’t resolve this. The mechanic also checked the wiring and said all the wiring looks good. Is there something that could be wrong with the actual check engine prompt itself?

Did the mechanic actually check the circuit for the speed sensor? Codes can’t tell you that a part is bad - only that there is a problem somewhere in the system. The speed sensor has a plug & wiring harness & wires running to & fro that need to be within certain voltage specs.

If you were to post the actual code (format P0123) it might be possible to get more specific suggestions.

I don’t know. I’ll ask him if he checked the circuit and get the code from him. Thank you!

Your mechanic needs to check the voltages on the wiring to the vehicle speed sensor, and to the engine computer, WHILE the engine is running. Instructions are in the repair manual.

Thanks, I’ll talk to him about this.

For those who helped me,here’s a follow-up to what happened–my check engine light suddenly went back off a few days after I posted the problem on Car Talk. Then it came back on for a couple of weeks, then for the past week it’s been off again. Next time I take my truck in for an oil change I’ll tell my mechanic about it. Very puzzling; however my truck is running fine as far as I can tell. Thanks again for responding to the post.