BMW Fault code 2C9F or P0161

1 Locate the electrical connector for the oxygen sensor with the heater circuit fault codes

2 Disconnect the electrical connector

3 Using a wiring diagram, identify the terminals for the heater circuit

4 Insert terminal test adapters into the heater circuit terminals

5 With your DVOM in Ohms, zero the meter, then connect it to the test adapters

6 Measure heater circuit resistance

The heater circuit on our subject vehicle is Open, indicating a faulty heater. The heater circuit should have no more than 60 kOhms.

7 To confirm a good reading, disconnect the second rear oxygen sensor connector

8 Insert terminal test adapters into the heater circuit terminals

9 With your DVOM in Ohms, zero the meter, then connect it to the test adapters

10 Measure heater circuit resistance

The sensor without a fault code has about 10 ohms resistance, indicating a good heater circuit.

11 To test the circuit with a scope, reconnect sensor electrical connectors

12 Then, insert backprobes into the heater circuit terminals and the sensor signal terminal

13 Connect the scope test leads to the backprobes

14 Connect the scope ground to chassis ground

15 Set channel A to 20v dc with 200 ms per div

16 Set channel B to 1v dc

17 Start and idle the vehicle

18 Example of Faulty Sensor

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.

19 Example of Good Sensor

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.