Oxygen Sensor Before Catalytic Converter, Checking Heater

Diagnostic Procedures

Oxygen Sensor before Catalytic Converter, Checking Heater

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

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• The Heated Oxygen Sensor (HO2S) (G39) fuse S-243 is 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 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 engine cover with air filter. Refer to the Repair Information.

Checking internal resistance

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





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

Specified value: 2.5 to 19.0 ohms (at approx. 20° C)

If the specified value was Not obtained:

- Replace the Heated Oxygen Sensor (HO2S) (G39).

If the specified value was obtained:

Checking voltage supply

- Start the engine.

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

Turn the ignition switch OFF.

Specified value: Battery voltage.





If the specified value was Not obtained:

- Check the wiring for an open, high resistance or short to Ground and replace any open fuse.

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

- If necessary, repair the faulty wiring connection.

If the specified value was obtained:

Checking Ground activation

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) ( J220). Refer to the Repair Information.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector terminal 3 to the Motronic Engine Control Module (ECM) (J220) electrical harness connector T121 terminal 5 for an open circuit.





Specified value: 1.5 ohms max.

If the specified value was Not obtained:

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

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

- If necessary, repair the faulty wiring connection.

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

- Replace the Motronic Engine Control Module (ECM) (J220). Refer to the Repair Information.

Final procedures

After the 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 Modes 01 - 09.

2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Modes 01 - 09.

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