Oxygen Sensor Before Catalyst, Checking

Oxygen Sensor (O2S) and oxygen sensor control before Three Way Catalytic Converter (TWC), checking


NOTE: When servicing terminals in harness connector of Heated Oxygen Sensor (HO2S), use only goldplated terminals.


Recommended special tools and equipment
- V.A.G 1526 portable multimeter or V.A.G 1715 multimeter
- V.A.G 1594 connector test kit
- Wiring diagram

Test requirements







- The respective fuses of Heated Oxygen Sensor (HO2S) -G39- must be OK.
- Battery voltage must be at least 11.5 volts.
- All electrical consumers like, for example, lights and rear window heating must be switched off.
- Parking brake must be engaged or else daylight driving lights will be switched on.
- 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 degrees C, Diagnostic mode 1: Check measuring values; PID 5, Coolant temperature. Refer to "Scan Tool Testing/Diagnostic Mode 1: Check Measuring Values".
Diagnostic Mode 1: Check Measuring Values

- Oxygen sensor heaters for oxygen sensors before catalytic converter must be OK,
Oxygen Sensor Heater For Oxygen Sensors Before Three Way Catalytic Converter (TWC), Checking

Function test
- Select the following output of oxygen sensor signals and check whether the specified values are being reached, Diagnostic mode 5: Check output of oxygen sensor signals. Refer to "Scan Tool Testing/Diagnostic Mode 5: Check Output of Oxygen Sensor Signals".
Diagnostic Mode 5: Check Output of Oxygen Sensor Signals

If specified values are obtained:
- End diagnosis and switch ignition off.

If specified values are not obtained:
- End diagnosis and let engine continue to run at idle.
- Check primary voltage.

Checking primary voltage

Vehicles with engine code AEB







- Disconnect 4-pin harness connector (brown) to Heated Oxygen Sensor (HO2S) -G39- (-1-).







- Use adapter cables from (VAG1594) to connect multimeter to terminals 3 + 4 (ECM connector) for voltage measurement.
- Start engine and measure primary voltage.
Specified value: 0.40 to 0.50 Volts
- Switch ignition off.

Vehicles with engine code ATW







- Disconnect 4-pin harness connector (black) to Heated Oxygen Sensor (HO2S) -G39- (-1-).







- Use adapter cables from V.A.G 1594 to connect multimeter to terminals 3 + 4 (ECM connector) for voltage measurement.
- Switch on ignition and measure primary voltage.
Specified value: 0.40 to 0.50 Volts
- Switch ignition off.

Continuation for all vehicles
If specified value is not obtained:
- Test oxygen sensor wires.

If specified value is obtained:
- Replace Heated Oxygen Sensor (HO2S) -G39-before catalytic converter, item 4 .
- Erase DTC memory of Engine Control Module (ECM), Diagnostic mode 4: Reset/erase diagnostic data.
- Generate readiness code. Refer to "Monitors, Trips and/or Drive Cycle/Readiness Code, Generating".
Monitors, Trips, Drive Cycles and Readiness Codes

Checking oxygen sensor wires
- Connect test box to control module wiring harness, connect test box for wiring test. Refer to "Scan Tool Connecting/Test Box, Connecting For Wiring Test".
Test Box, Connecting For Wiring Test

Vehicles with engine code AEB







- Disconnect 4-pin harness connector (brown) to Heated Oxygen Sensor (HO2S) -G39- (-1-).







- Check wires between test box and 4-pin connector (to control module) for open circuit according to wiring diagram.
Terminal 3 + socket 25
Terminal 4 + socket 26
Wire resistance: max: 1.5 Ohms

Vehicles with engine code ATW







- Disconnect 4-pin harness connector (black) to Heated Oxygen Sensor (HO2S) -G39- (-1-).







- Check wires between test box and 4-pin connector (to control module) for open circuit according to wiring diagram.
Terminal 3 + socket 51
Terminal 4 + socket 70
Wire resistance: max: 1.5

Vehicles with engine code AUG, AWM







- Disconnect 6-pin harness connector (black) to Heated Oxygen Sensor (HO2S) -G39- (item no. 1).







- Check wires between test box and 6-pin connector (to control module) for open circuit according to wiring diagram.
Terminal 1 + socket 70
Terminal 2 + socket 71
Terminal 4 + socket 05
Terminal 5 + socket 51
Terminal 6 + socket 52
Wire resistance: max: 1.5 Ohms

Continuation for all vehicles
- Also check wires for short circuit to B+ and Ground (GND).
Specified value: infinity Ohms

If no malfunctions are found in wires:
- Replace Heated Oxygen Sensor (HO2S) -G39- before catalytic converter, item 4 .