In this article and video we will cover how to diagnose BMW Fault code 2C9F or P0161 - Oxygen sensor behind catalytic converter 2, signal using a DVOM and an Opus IVS PicoScope. This test can be used on most vehicles with a duty cycle controlled unit.
The heater circuit on our subject vehicle is Open, indicating a faulty heater. The heater circuit should have no more than 60 kOhms.
The sensor without a fault code has about 10 ohms resistance, indicating a good heater circuit.
The pattern shows a good duty cycle control signal coming from the DME. This proves the vehicle side of the circuit, including DME is good, and confirms the diagnosis of a bad heater circuit within the O2 sensor.
You can also see that the sensor voltage from the bas sensor hovers around 400mv. This is because the heater is not working, and the sensor is cold.
The good sensor sits around 700mv because the sensor is at the correct temperature to operate properly.
The steady voltage also is indicative of a properly working catalytic converter.