Oxygen Sensor Before Catalytic Converter, Checking

Oxygen Sensor Before Catalytic Converter, Checking

• Use only gold-plated terminals when servicing terminals in the electrical harness connector of the Heated Oxygen Sensor (HO2S) (G39).

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• The Heated Oxygen Sensor (HO2S) (G39) fuse OK.






• The Oxygen Sensor (O2S) Heater (G39) before catalytic converter 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 connections between engine/transmission/chassis OK.

• Exhaust system between catalytic converter and cylinder head properly sealed.

• Coolant Temperature at least 80 °C.

• Ignition switched OFF

Test procedure

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

Start diagnosis

- 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 "Monitor ID 01: Oxygen sensor monitoring before catalytic converter".

- Select "Test ID 131 ($89 for later models): Dynamic test".

- Check the specified values at idle.

Refer to Mode 6 table: => [ Monitor-ID $01: Oxygen Sensor
Monitor Bank 1 - Sensor 1 ] Diagnostic Modes 01 - 09 for values.

If the specified value was obtained:

- Switch the ignition OFF. Fault is not active at this time. Check for poor connections, loose terminals and harness check.

If the specified value was Not obtained:

Checking primary voltage

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






- Turn the ignition switch ON.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector - 1 - terminals 1 to 2 for voltage.






Specified value: 0.400 to 0.500 Volts

- Turn the ignition switch OFF.

If the specified value was obtained:

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

If the specified value was Not obtained:

Checking wiring

- Remove the Engine Control Module (ECM) (J623). Refer to the Repair Information.

- Using a multimeter, check the Heated Oxygen Sensor (HO2S) (G39) electrical harness connector terminals to the Engine Control Module (ECM) (J623) electrical harness connector T94 terminals 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 or short to 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 Not OK:

- Replace the Engine Control Module (ECM) (J623). 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.