Make sure the iO2SS is properly powered. Measure the unit's power input (red wire) when the engine is on; you should measure >12V.
Ensure the iO2SS's signal outputs (green wires) are connected to the appropriate oxygen sensor signal wires leading to the PCM.
Verify all other wiring and check all solder joints or crimp connectors for loose connections, frayed wires, or corrosion.
If using an iO2SS in automatic (signal augmentation) mode, your oxygen sensor may be damaged. Consider using the iO2SS in manual (signal generation) mode instead.