Heated Oxygen Sensor (HO2S) G39, (HO2S) 2 G108 and (O2S) Regulation Before Catalytic Converter, Checking

Heated Oxygen Sensor (HO2S) G39 / Heated Oxygen Sensor (HO2S) 2 G108 And Oxygen Sensor Regulation Before Catalytic Converter, Checking

The following procedure is used to diagnose both Heated Oxygen Sensor (HO2S) G39 and Heated Oxygen Sensor (HO2S) 2 G108

NOTE: Use only gold-plated terminals when servicing terminals in the electrical harness connectors of the Heated Oxygen Sensor (HO2S) G39 and Heated Oxygen Sensor (HO2S) 2 G108.

Special tools, testers and auxiliary items required
- Multimeter.
- Wiring diagram.

Test requirements
- The Heated Oxygen Sensor (HO2S) G39 and Oxygen Sensor (O2S) 2 Heater Z28 Engine Electronics Fuse S282 OK.
- The Oxygen Sensor (O2S) Heater Z19 and Oxygen Sensor (O2S) 2 Heater Z28 before catalytic converter OK.
- The Fuel Pump (FP) Relay J17 OK.
- Battery voltage at least 12.5 volts.
- All electrical consumers such as, lights and rear window defroster, switched off.
- A/C switched off.
- Ground (GND) connections between engine/transmission/chassis OK.
- Exhaust system between catalytic converter and cylinder head properly sealed.
- Coolant Temperature at least 80 degrees C.

Function test
- Connect scan tool.
- Start the engine and let it run at idle.
- Select "Diagnostic mode 6: Check test results of components that are not continuously monitored".
- Select "Test-ID 01: Oxygen sensor monitoring before catalytic converter".
- Select "Test-ID 131: Dynamic test".
- Check the specified values at idle.

Dynamic test Specified value
min. max.
Normal operation 0.35009 -
Reduced threshold upon suspicion of 0.39990 -
malfunction in catalytic converter

If specified values are obtained:
- End diagnosis and switch the ignition off.

If the specified values are not obtained:

Procedure for accessing the Heated Oxygen Sensor (HO2S) 2 G108 electrical harness.







- Remove the left side engine compartment cover - 1 -.
- Remove the coolant expansion tank screw - arrow -.







- Disconnect the electrical connector from the Engine Coolant Level (ECL) Warning Switch F66 and set aside coolant reservoir with coolant hoses - 1 - and - 2 - connected.







- Disconnect the Heated Oxygen Sensor (HO2S) 2 G108 electrical harness connector - 4 -.

Procedure for accessing the Heated Oxygen Sensor (HO2S) G39 electrical harness.







- Remove the cover on the right side of the engine compartment - 1 -.







- Remove the air intake duct - arrows -.







- Disconnect the Heated Oxygen Sensor (HO2S) G39 electrical harness connector - 1 -.

Checking primary voltage







- Using a multimeter, check the Heated Oxygen Sensor (HO2S) electrical harness connector terminals 1 to 5 for voltage.
Specified value: 0.400 to 0.500 Volts
- Switch the ignition off.

If the specified value was not obtained:
- Replace the Heated Oxygen Sensor (HO2S) G39 or the Heated Oxygen Sensor (HO2S) 2 G108.


If the specified value was obtained:

Checking wiring
If the manufacturers test box is being used. Perform the following step.
- Install the test box.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) J623.







- Using a Multimeter, check the Heated Oxygen Sensor (HO2S) G39 or the Heated Oxygen Sensor (HO2S) 2 G108 electrical harness connector terminals to the Motronic Engine Control Module (ECM) J623 electrical harness connector T121 terminals for an open circuit.

Heated Oxygen Sensor (HO2S) Motronic Engine Control Module
G39 electrical harness (ECM) J623 electrical connector
connector terminals T121 terminals or test box sockets
1 70
2 71
5 51
6 52

Heated Oxygen Sensor (HO2S) Motronic Engine Control Module
2 G108 electrical harness (ECM) J623 electrical connector
connector terminals T121 terminals or test box sockets
1 13
2 14
5 12
6 15

Specified value: 1.5 Ohms Max.

If the specification was not obtained:
- Check the wiring for a short circuit to each other, Battery (+), and Ground (GND).
- Check the electrical harness connector for damage, corrosion, lose or broken terminals.
- If necessary, repair the faulty wiring connection.

If no malfunction is found in the wiring:
- Replace the Motronic Engine Control Module (ECM) J623.

Final procedures
After repair work, the following work steps must be performed in the following sequence:
1. Check the DTC memory. Diagnostic Mode 03 - Read DTC Memory
2. If necessary, erase the DTC memory. Diagnostic Mode 04 - Erase DTC Memory
3. If the DTC memory was erased, generate readiness code.

- End of diagnosis.