In this article and video we will cover how to diagnose BMW Fault code 2C78 or P0160 - Oxygen sensor behind catalytic converter 2, signal using a DVOM and an Autologic PicoScope. This test can be used on most vehicles with a duty cycle controlled unit.
BMW Fault code 2C78 or P0160
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.