Some ECUs use the o2 sensors to sniff the exhaust gases to make sure the Secondary Air Injection (SAI) pump is running. However, this ECU is not that clever. This ECU is only measuring return voltage. The pump is hidden behind an automotive relay so the only thing the ECU can see is the coil in the relay. You can easily fool the ECU by bridging the SAI connector with a 100-200 Ohm resistor. It’s not too critical what value you use provided it adds a little resistance.