Oxygen Sensor Before Catalytic Converter, Checking

Observe all safety precautions: SAFETY PRECAUTIONS 

View clean working conditions: CLEAN WORKING CONDITIONS 

Prior to repair work, perform a preliminary check to verify the condition. Refer to PRELIMINARY CHECK .

Use only gold-plated terminals when servicing any component with gold-plated electrical harness connector terminals.

For wiring diagrams, component locations, and connector views, Refer to the applicable wiring diagram.

Special tools and workshop equipment required 

• Multimeter
• Wiring diagram

Test requirements 

• Fuse for Heated Oxygen Sensor (HO2S) -G39- 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.

Function test 

-- Connect the scan tool.

-- Start engine and let it run at idle.

-- Select Diagnostic mode 6.

-- Select "Test-ID 01: Heated Oxygen Sensor". Refer to test DIAGNOSTIC MODE 06 - READ TEST RESULTS FOR SPECIFIC DIAGNOSTIC FUNCTIONS .

-- Switch ignition off.

If the Monitor Failed:

Checking heater resistance

-- Disconnect the Heated Oxygen Sensor (HO2S) --G39-- electrical harness connector -3- at the engine bulkhead (left side).

-- Using a multimeter, measure the resistance of the heater from terminal 3 to terminal 4 of the Heated Oxygen Sensor (HO2S) --G39-- connector.

Specified value: 2 - 4 Ω with the exhaust system at room temperature

If the specified value was Not obtained:

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

If the specified value was obtained:

Checking heater voltage 

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

-- Switch ignition on.

-- Using a multimeter, measure the voltage from terminal 4 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near battery voltage

-- Switch ignition off.

If specified value is Not obtained:

-- Check for an open, short to ground or high resistance between terminal 4 of the Heated Oxygen Sensor (HO2S) --G39-- and fuse 21 in Fuse Box B.

-- If necessary, repair the wiring connection.

If the specified value was obtained:

Checking HO2S voltage

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

-- Switch ignition on.

-- Using a multimeter, measure the voltage from terminal 3 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near 4.76 V

-- Using a multimeter, measure the voltage from terminal 1 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near 2.85 V

-- Using a multimeter, measure the voltage from terminal 5 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near 2.85 V

-- Using a multimeter, measure the voltage from terminal 2 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near 2.50 V

-- Using a multimeter, measure the voltage from terminal 6 of the Heated Oxygen Sensor (HO2S) --G39-- harness connector to ground.

Specification: Near 2.93 V

-- Switch ignition off.

If any of the specified values were Not obtained:

Checking wiring 

-- Remove the Motronic Engine Control Module (ECM) -J623-. Refer to the service information for removal and installation procedures.

-- Using a multimeter, check the Heated Oxygen Sensor (HO2S) --G39-- electrical harness connector terminals to the Motronic Engine Control Module (ECM) -J623- electrical connector T94 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
Pin 1	77
Pin 2	56
Pin 3	73
Pin 5	55
Pin 6	78

Specified resistance 1.5 Ω max.

If the specification is Not obtained:

-- Check the wiring for high resistance, an open or short circuit to Battery or ground.

-- If necessary, repair the wiring connection.

If no malfunctions are found in wiring and the voltage was Not OK:

-- Replace the Motronic 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 .
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 .