After searching through the manual, you need to do the following:

1) Make sure the sensor is mounted fully and securely. I assume that means flush.

2) Check the exhaust for leaks

3) You may need a new catalytic converter, it will throw the same error code. Not sure how to test one though.

If everything else is good, no exhaust leaks, sensor seated properly, wires not touching exhaust and are free from damage, you either need to 1) change the sensor (again) or have it tested. There is a tool called a TAC2 that is from Isuzu, but I don't know how to get one, that runs the test. Or, 2) replace the catalytic converter. That last part sounds expensive.