2002 Jackaroo o2 sensor problems

I have a 2002 Jackaroo 3.5L V6 manual with 55,000 klms on the clock. The "CHECK ENGINE" light came on a couple of weeks ago, took it into Holden, the diagnosed it as having an O2 sensor prob, changed it, about a week later the "CHECK ENGINE" light is back on again ... for a couple of days ... then back off again for a couple of days ... then back on again ... etc

When I took it back to Holden they just said the connector from the O2 sensor wire to the wiring harness was a bit loose ... so they tightened it (so they say) ... Well, the "CHECK ENGINE" light was out when I got it back ... but now it's back into the on-for-a-bit / off-for-a-bit pattern ...

I've seen some posts to suggest that the "CHECK ENGINE" light is effectively a useless indicator (unless it's flashing then it means trouble) ...

I've also seen some posts which suggest possibly the wrong O2 sensor was replaced. I can only find one sensor on my model (before the cat). Where's the other one ?

Anyways, if anyone has any suggestions (please none suggesting I buy another make/model) I would be gratefull.

Glen