High O2 sensor voltage does indicate rich. A working O2 sensor and PCM will have the voltage oscillating around ~450mV but only with the sensor hot and the engine off idle. I would: Check the O2 sensor operation away from idle eg. at light load operation eg. normal driving with gentle accel. and cruise. Perform a check for any fault codes. The O2 sensor is a very temperature sensitive device with readings taken when it is below 570º being inaccurate. As a result, the engine controller stays in Open Loop mode. To counteract this problem, the O2 sensor is equipped with a heating element to heat the ceramic sensing element.
This can be the narrow band O2 sensor (O2 INPUT) on the ECU (pin 19), or one of the NEMU ADC inputs. If "O2 INPUT" selected, the 0-5V wideband output requires a divide by 2 voltage divider before the narrow band O2 sensor ECU input. TYPE - Specify the type of wideband sensor connected. FILTER - This sets the input smoothing filter count (max=7).