Oxygen Sensor Heater Before Catalytic Converter, Checking

Special Tools And Workshop Equipment Required 

• Multimeter.
• Wiring diagram.

Test Requirements 

• The Heated Oxygen Sensor (HO2S) -G39- fuse 21 in Fuse Panel B 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 

Start Diagnosis 

-- Remove the engine cover with air filter. Refer to the appropriate Service Information.

Checking internal resistance

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

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

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

If the specified value was Not obtained: 

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

If the specified value was obtained:

Checking voltage supply

-- Turn the ignition switch ON

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

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 (GND) 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) -J623-. Refer to the appropriate Service Information.

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

Heated Oxygen Sensor (HO2S) -G39- electrical harness connector terminal	Motronic Engine Control Module (ECM) -J623- electrical connector T94 terminal or test box socket
3	73

Specified Value: 1.5 Ω Max. 

If the specified value was Not obtained: 

-- Check the wiring for an open, 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 if the voltage supply was OK: 

-- Replace the Motronic Engine Control Module (ECM) -J623-. Refer to the appropriate Service Information.

-- Install the engine cover with air filter. Refer to the appropriate Service 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 .
2. If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
3. If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .