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Oxygen Sensors and Sensor Regulation Before Catalytic Converter, Checking






Diagnostic Procedures

Oxygen Sensors and Sensor Regulation before Catalytic Converter, Checking

When servicing terminals in harness connector of oxygen sensors, use only gold-plated terminals.

The oxygen sensors before catalytic converter have a constant regulation and can be differentiated from the oxygen sensor behind catalytic converter via a 6-pin connector.

Special tools, testers and auxiliary items required

Multimeter (VAG1526) or multimeter (VAG1715)

Connector test kit (VAG1594)

Wiring diagram

Test requirements

Fuses in E-box (in plenum chamber, left) must be OK:





Motronic Engine Control Module (ECM) Power Supply Relay (J271) must be OK, check:

Battery voltage must be at least 11.5 volts.

All electrical consumers such as, for example, lights and rear window defroster must be switched off.

Parking brake must be engaged or else daylight driving lights will be switched on.

For vehicles with automatic transmission, selector lever must be in P position.

If vehicle is equipped with an A/C system, it must be switched off.

Ground (GND) connections between engine and chassis must be OK.

Exhaust system must be properly sealed between catalytic converter and cylinder head.

Coolant temperature must be at least 80° C , Diagnostic mode 1: Check measured values; PID 5, Coolant temperature.

Oxygen sensor heaters for oxygen sensors before catalytic converter must be OK, check --> [ Oxygen Sensor Heater before Catalytic Converter, Checking ] Oxygen Sensor Heater Before Catalytic Converter, Checking.

Function test

- Select the output of oxygen sensor signals and check whether the specified values are being reached , Diagnostic mode 5: Check output of oxygen sensor signals.

- End diagnosis and switch ignition off.

If specified values are not obtained:

- Check oxygen sensor wires according to wiring diagram.

Checking oxygen sensor wires

- Connect test box to control module wiring harness , connect test box for wiring test.

- Disconnect 6-pin harness connector to the respective oxygen sensor:





Heated Oxygen Sensor (HO2S) (G39), bank 1, 6-pin harness connector - 2 -,

Heated Oxygen Sensor (HO2S) 2 (G108), bank 2, 6-pin harness connector - 4 -.

Bank 1 = Cylinder bank 1, in direction of travel, right

Bank 2 = Cylinder bank 2, in direction of travel, left

- Check wires between test box and 6-pin connector to Engine Control Module (ECM) for open circuit according to wiring diagram.





Wire resistance: max. 1.5 ohms





- Also check wires for short circuit to B+ or Ground (GND).

Specified value: infinite ohms

If no malfunctions are found in wires:

- Replace the respective oxygen sensor:

Bank 1 Heated Oxygen Sensor (HO2S) (G39) ,

Bank 2 Heated Oxygen Sensor (HO2S) 2 (G108).

- Erase DTC memory of Engine Control Module (ECM) , Diagnostic mode 4: Reset/erase diagnostic data.

- Generate readiness code.