The stock o2 sensor is a wideband

[CTScott]

ilikerice sent me a PM with a question about connecting a gauge to the OEM primary sensor, so I figured I'd answer it in here instead of by PM.

The stock AFR sensor is a typical heated platinum coated zirconia A/F sensor. These sensors require a considerable amount of work by the device monitoring them. The heater within the sensor is used to increase its sensitivity when the exhaust gas is cold, but if it gets too hot it also has accuracy issues, so the monitoring device must constantly adjust the duty cycle of the heater. It also maintains a constant voltage across the A1A+ to A1A- terminals of the sensor. It measures the current draw, rather than voltage change of the sensor, so if your were to measure the voltage on the A1A+ or A1A- pins of the sensor, they would remain constant. So, since reading the sensor requires closed loop control, it would be almost impossible to run two gauges on one sensor.

Using the Toyota diagnostics messaging (a step above simple OBDII) that I reverse engineered from the TechStream tool, I read and display the realtime AFR on my YarGauge.

Some vehicles support an OBDII PID for AFR (which allows displaying it as a X-gauge on the scangauge), but due to the communication frequency limitations of OBDII this tends lag significantly, making it not very useful.