Land Rover Repair: 99 Discovery II, land rover dealer, oxygen sensors

Question
Hello, and thanks in advance for your help!  I have a 99 Discovery II with 150000 miles and the check engine light and the three amigos on.  First visit to a local mechanic I was told to start with it needed a new converter and new O2 sensors.  I replaced these with a Davico converter and SNG O2 sensors and returned to the garage -light is still on and still getting codes on the O2 sensors. The mechanic checked all wires from sensors back to the engine computer and everything checked ok-thought it could possibly be the computer and referred me to a Land Rover dealer for further diagnostics.  Land Rover dealer runs diagnostic check and receives codes for all four oxygen sensors and thinks the problem is in the aftermarket parts.  They say the o2 sensors work fine for numerous cycles and then suddenly jump out of range and then return to cycling correctly.  This jump is sending the code to the computer and activating the check engine light and is possibly caused by different parameters between the sensors and computer?  They suggest replacing the converter and sensors with Land Rover parts as a starting point to fix the vehicle. They also retrieved a purge valve code, and abs left front sensor.  The dealer basically said it would cost more than the vehicle is worth to fix($3000-$4000).  The truck is very sound and runs good but will not pass state inspection with the check engine light on. I can not get past this first problem to address the other issues. Do you have any suggestions and have you encountered this problem with aftermarket parts before? Any thoughts and advice would be much appreciated!

Answer
My instinct is to agree with the dealer on this.  I have seen many problems like yours caused by sub-par aftermarket parts.  If you truck was in our shop that's where I would start.  However, there may be something more going on and I'd caution that it may end up costly to fix.  But that's a reality we face every day at Robison Service and people for the most part fix the vehicles and end up happy with the result.  For satisfied motorists, there's more to ownership than a trade of cash value versus repair cost.  If that's all there is for you and this particular truck, then maybe it's time to trade.

Good luck with it
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John Elder Robison