First, i would clear all the codes and tables - disconnect the batt for 15 - 20 mins and leave a door open....
Then connect it back up and take it for a drive around for a few days and pull the codes off it again.
If you still get the rich code,
go to your local wreckers and pick up a 2nd hand O2 sensor off an EA to EL falc and try it, see if you still get the rich code, alternatively you could borrow one off a mate if you know someone that has an E series.
If it fixes it either keep the 2nd hand one (from wreckers) or go out and get a new one.
If you still get the rich code, then you need to look at MAP or ECT or ACT sensors, they might be slightly out of spec, but not enough for the ecu to give it a fault status.
Also, you can make your own reverse polarity test light by using a red LED, a green LED, and a 1k resistor, and a some alligator clips and a few feet of wire -you can buy these from jaycar / dick smith for around $8 total.
see the attached file...
* note that one LED is in the reverse direction as the other.