Oxygen Sensor Heater Before Catalytic Converter, Checking

Diagnostic Procedures

Oxygen Sensor Heater before Catalytic Converter, Checking

Oxygen Sensor Heater 1

• When servicing electrical harness connector terminals for the Heated Oxygen Sensor (HO2S) (G39), use only gold-plated terminals.

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• Fuse (SC6) OK.

• Battery voltage at least 12.5 volts.

• All electrical consumers such as, lights and rear window defroster, switched off.

• Vehicles with automatic transmission, shift selector lever into position "P" or "N".

• A/C switched off.

• Ground (GND) connections between engine/transmission/chassis OK.

• Ignition switched off.

Test procedure

- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check.

Start diagnosis

- Remove the rear engine cover - arrows -.





- Remove the vacuum lines - 3 - and - 4 - from the air guide hose.

- Disconnect the Mass Air Flow (MAF) Sensor (G70) electrical harness connector - 1 -.

- Loosen the hose clamps - 2 - and - 5 - and remove the air guide hose.





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





Checking internal resistance

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) terminals 3 to 4 for resistance.





Specified value: 2.5 to 10.0 ohms (at approx. 20° C)

If the specification was not obtained:

- Replace Heated Oxygen Sensor (HO2S) (G39). Refer to Repair Information.

If the specification was obtained:

Checking voltage supply

Crank the engine.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector terminal 4 to Ground (GND) for voltage.









Specified value: Battery voltage.

Switch the ignition off.

If the specification was not obtained:

- Check the electrical harness connector for damage, corrosion, lose or broken terminals.

- Check the wiring from the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector terminal 4 to the Engine Component Power Supply Relay (J757) socket 2/87 for a short circuit to each other, Ground (GND), or an open circuit.

- If necessary, repair the faulty wiring connection.

If the specification was obtained:

Checking Ground (GND) activation

If the manufacturers test box is being used. Perform the following step.

- Install the test box. Refer to Repair Information.

If the manufacturers test box is not being used. Perform the following step.

- Remove the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94 for an open circuit.









Specified value: 1.5 ohms max.

If the specification was not obtained:

- Check the wiring for a short circuit to each other, Battery (+), Ground (GND) or an open circuit.

- If necessary, repair the faulty wiring connection.

If no malfunction is detected in the wiring and if the voltage supply was OK:

- Replace the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

Assembly is performed in the reverse order of removal.

Final procedures

After repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Mode 03 - Read DTC Memory.

2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Mode 04 - Erase DTC Memory.

3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.

- End of diagnosis.

Oxygen Sensor Heater 2

• When servicing electrical harness connector terminals for the Heated Oxygen Sensors, use only gold-plated terminals.

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• Fuse (SC6) OK.

• Battery voltage at least 12.5 volts.

• All electrical consumers such as, lights and rear window defroster, switched off.

• Vehicles with automatic transmission, shift selector lever into position "P" or "N".

• A/C switched off.

• Ground (GND) connections between engine/transmission/chassis OK.

• Ignition switched off.

Test procedure

- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check.

Start diagnosis

- Remove the rear engine cover - arrows -.





- Remove the vacuum lines - 3 - and - 4 - from the air guide hose.

- Disconnect the Mass Air Flow (MAF) Sensor (G70) electrical harness connector - 1 -.

- Loosen the hose clamps - 2 - and - 5 - and remove the air guide hose.





- Disconnect the Heated Oxygen Sensor (HO2S) 2 (G108) electrical harness connector - 3 -.





Checking internal resistance

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) 2 (G108) terminals 3 to 4 for resistance.





Specified value: 2.5 to 10.0 ohms (at approx. 20° C)

If the specification was not obtained:

- Replace Heated Oxygen Sensor (HO2S) 2 (G108). Refer to Repair Information.

If the specification was obtained:

Checking voltage supply

Crank the engine.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) 2 (G108) electrical harness connector terminal 4 to Ground (GND) for voltage.









Specified value: Battery voltage.

Switch the ignition off.

If the specification was not obtained:

- Check the electrical harness connector for damage, corrosion, lose or broken terminals.

- Check the wiring from the Heated Oxygen Sensor (HO2S) 2 (G108) electrical harness connector terminal 4 to the Engine Component Power Supply Relay (J757) socket 2/87 for a short circuit to each other, Ground (GND), or an open circuit.

- If necessary, repair the faulty wiring connection.

If the specification was obtained:

Checking Ground (GND) activation

If the manufacturers test box is being used. Perform the following step.

- Install the test box. Refer to Repair Information.

If the manufacturers test box is not being used. Perform the following step.

- Remove the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) 2 (G108) electrical harness connector to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94 for an open circuit.









Specified value: 1.5 ohms max.

If the specification was not obtained:

- Check the wiring for a short circuit to each other, Battery (+), Ground (GND) or an open circuit.

- If necessary, repair the faulty wiring connection.

If no malfunction is detected in the wiring and if the voltage supply was OK:

- Replace the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

Assembly is performed in the reverse order of removal.

Final procedures

After repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Mode 03 - Read DTC Memory.

2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Mode 04 - Erase DTC Memory.

3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.

- End of diagnosis.