4 trouble codes on 97 Nissan Maxima: 0325, 0400, 0500, 0600

Originally posted to Nissan Forums:

My check engine light recently came back on in my 1997 Nissan Maxima SE. I took it to AutoZone, an employee checked for codes, and found the following:

0325: Knock Sensor Circuit Malfunction

0400: EGR function – no/low flow detected

0500: Vehicle speed sensor circuit fault

0600: AT Communication error

I’ve seen at least 3 of these 4 codes previously (possibly all 4; my memory fails me) when dealing with a recurring issue: My speedometer would sporadically not activate (for lack of a better word) when driving. The issue started occurring more frequently until I finally took it to a mechanic, who gave me two options: A quick but likely temporary fix at a lower cost or a more expensive but permanent fix. I opted for the less expensive fix and didn’t experience the issue again for several months.

I started having the same issue again recently (speedometer not activating), but smacking the dashboard directly above the speedometer would sometimes temporarily force the speedometer to work again.

I haven’t noticed the car handling any differently and haven’t had the speedometer issue in the last week or so, but I’m concerned that the AutoZone employee found 4 trouble codes. I’m not that concerned about driving around town, but I sometimes visit family out of town and I don’t want to take my Maxima on the highway without first knowing that it’s safe.

As for many others in this economy (and terrible, terrible job market), money is tight right now, so I don’t want to spend more than absolutely necessary to resolve these issues once and for all; however, I also don’t want to neglect my or my family’s safety.

What would you all advise?