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 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 

Start diagnosis 

-- Remove the engine cover with air filter. Refer to Repair 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+4 for an open circuit.

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

If the specification was not obtained:

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

If the specification is obtained:

Checking voltage supply 

-- Using a Multimeter, check the Heated Oxygen Sensor (HO2S) -G39- electrical harness connector terminal 4 to the Engine Component Power Supply Relay -J757- terminal 2 for voltage.

Heated Oxygen Sensor (HO2S) -G39- electrical harness connector terminal	Engine Component Power Supply Relay -J757- electrical harness connector terminal
4	2

Specified value: Battery voltage.

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 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) -J220-.

-- 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 T94 terminal 5 for an open circuit.

Heated Oxygen Sensor (HO2S) -G39- electrical harness connector terminal	Motronic Engine Control Module (ECM) -J220- electrical connector T94 terminal or test box socket
3	5

Specified value: 1.5 Ω 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 detected in the wiring and if the voltage supply was OK:

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

-- Install the engine cover with air filter. Refer to 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 .
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 .

-- End of diagnosis.