Originally Posted by
BlackStealth
Disclaimer: I cannot confirm this information as I don't have a GTO service manual.
I think the GTO ECU uses the unused second O2 sensor wire for monitoring an Exhaust temperature sensor (0-5v) at the catalytic converter. When the GTO ECU determines it's too hot, it lights up the catalytic converter light (pin 6, which is on our US ECU is EGR solenoid). So now with the 99 flash ECU in place of the GTO ECU, whenever it tries to operate the EGR solenoid it is triggering these false positive catalytic converter light. To fix this light, just remove the light bulb, or cut the wire at pin 6 because I don't think the GTO's have an EGR solenoid.