Codes 34 and 37
 

My '92 had been in the shop since November 14th, and I FINALLY got it back today. It was in for new cam and crank seals, a water pump, timing belt, thermostat, and all of the underdrive pulleys. So I was extremely excited to get her back today, and everythign was great, except after driving about 5 miles, my Check engine Light came on for about a minute, and then went off. So I continued on a few more miles, and after parking for a few minutes, and then re-starting her, I got another Check engine light. I drove like that for the rest of the day (a total of about 20 miles, with constant stops in town, and actually parking about 5 or 6 times) and no more CEL. But I pulled the codes when I got home, and I got code 34 (EGR solenoid valve inoperative) and a 37 (02 sensor Left hand side inoperative). So my questions is, since those codes wont come back, was it possibly just a fluke? It had every opportunity in the world to throw those codes again, I even drove it pretty hard to try to get the CEL to come back on, but got nothing. I have cleared the codes, and am waiting for them to come back. If these both do need fixing, what does that require, and can I do it myself?

Perhaps the timing belt skipped?

Since the battery should have been disconnected with this type of work, it sounds to me the ECU was just readjusting itself to your engine specifics. The O2 sensor is a cinch as long as your exhaust at the CC isn't rusted up. The EGR solenoid can be a little hard getting to.

I think it's just a readjustment fluke anyway, and neither component is going to cause any collateral damage to anything else. The 02 sensor will make your gas mileage go to dirt, and of course both can mess up an emissions check. but this is stuff you already know.