Oxygen Sensors Before Catalytic Converter, Checking Heater

Oxygen Sensors before Catalytic Converter, Checking Heater

The following procedure is used to diagnose Oxygen Sensor (O2S) Heater Z19 and Oxygen Sensor (O2S) 2 Heater Z28.

NOTE: When servicing electrical harness connector terminals for the Heated Oxygen Sensor (HO2S) G39 , use only gold-plated terminals.

Special tools, testers and auxiliary items required
- Multimeter.
- Wiring diagram.

Test requirements
- The Oxygen Sensor (O2S) Heater Z19 and Oxygen Sensor (O2S) 2 Heater Z28 Engine Electronics Fuse S282 OK.
- The Fuel Pump (FP) Relay J17 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.

Procedure for accessing Oxygen Sensor (O2S) 2 Heater Z28 electrical harness.







- Remove the left side engine compartment cover - 1 -.
- Remove the coolant expansion tank screw - arrow -.







- Disconnect the electrical connector from the Engine Coolant Level (ECL) Warning Switch F66 and set aside coolant reservoir with coolant hoses - 1 - and - 2 - connected.







- Disconnect the Oxygen Sensor (O2S) 2 Heater Z28 for Heated Oxygen Sensor (HO2S) 2 G108 electrical harness connector - 4 -.

Procedure for accessing Oxygen Sensor (O2S) Heater Z19 electrical harness.







- Remove the cover on the right side of the engine compartment - 1 -.







- Remove the air intake duct - arrows -.







- Disconnect the Oxygen Sensor (O2S) Heater Z19 for Heated Oxygen Sensor (HO2S) G39 electrical harness connector - 1 -.

Checking internal resistance
- Using a multimeter , check the Heated Oxygen Sensor (HO2S) terminals 6 to 3 for an open circuit.

Specified value: 2.5 to 10.0 Ohms (at approx. 20 degrees C)

If the specification was not obtained:
- Replace the Heated Oxygen Sensor (HO2S) G39 or the Heated Oxygen Sensor (HO2S) 2 G108.

If the specification was obtained:

Checking voltage supply







- Using a multimeter , check the Heated Oxygen Sensor (HO2S) electrical harness connector terminal 3 to ground (GND) for voltage.

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.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) J623.
- Using a multimeter , check the Heated Oxygen Sensor (HO2S) electrical harness connector to the Motronic Engine Control Module (ECM) J623 electrical harness connector T121 for an open circuit.

Heated Oxygen Sensor (HO2S) Motronic Engine Control Module (ECM)
G39 electrical harness J623 electrical connector T121 terminal
connector terminal or test box socket
4 5

Heated Oxygen Sensor (HO2S) 2 Motronic Engine Control Module (ECM)
G108 electrical harness J623 electrical connector T121 terminal
connector terminal or test box socket
4 4

Specified value: 1.5 Ohms 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) J623.
- Install the engine cover with air filter.

Final procedures
After repair work, the following work steps must be performed in the following sequence:
1. Check the DTC memory. Diagnostic Mode 03 - Read DTC Memory
2. If necessary, erase the DTC memory. Diagnostic Mode 04 - Erase DTC Memory
3. If the DTC memory was erased, generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes