Oxygen Sensor After Catalyst, Checking
Heated Oxygen Sensor (HO2S) And O2S Control After TWC, Testing
NOTE: Oxygen sensor control behind catalytic converter overrules oxygen sensor control before catalytic converter and is responsible for control correction.
It corrects slight changes in the mixture (i.e. enrichment) via the oxygen
sensor before catalytic converter by holding oxygen sensor control before
catalytic converter at its higher or lower point for a specific time (dwell
time). If this time is in the positive range (i.e. 50 ms), mixture is shifted in the -enrich- direction. If this time is in the negative range (i.e. 50 ms),
mixture is shifted in the -lean- direction.
The 4-pin harness connector for the oxygen sensor behind catalytic converter and oxygen sensor heating is under a cover -2- at vehicle floor.
Special tools and equipment
- VAG1526A
- VAG1594A
- VAG1598/31
- VAG1788/10 RPM adjuster
- VAS5051 with VAG5051/1
or
- VAG1551 with VAG1551/3B
Test requirements:
- Road test performed and DTC memory erased.
- Coolant temperature at least 80 degree C.
Test sequence
- Connect VAS5051 tester or VAG1551 scan tool and select the control module for engine electronics using "address word" 01. Connecting VAS 5051/VAG1551 Scan Tool
Engine must run at idle for this.
Indicated on display:
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -4- to select "Initiate basic setting" and press -Q- button to confirm input.
NOTE: During basic setting, the Evaporative Emission (EVAP) canister purge regulator valve (EVAP valve -N80-) is closed and the A/C compressor is switched off
Indicated on display:
System in Basic Setting Q
Input display group number XXX
- Press buttons -0-, -3- and -4- to select "display group number 034" and press -Q- button to confirm input.
Indicated on display:
System in Basic Setting 34 ->
1 2 3 4
- Use VAG1788/10 RPM adjuster to adjust engine speed to between 2800 and 3200 RPM.
- Continue test as soon as display field 2 indicates an exhaust temperature of more than 350 degree C.
NOTE: This process can take a few minutes.
- Press -C- button.
- Use VAG1788/10 RPM adjuster to adjust engine speed to between 1800 and 2200 RPM.
Indicated on display:
System in Basic Setting Q
Input display group number XXX
- Press buttons -0-, -3- and -0- to select "display group number 030" and press -Q- button to confirm input.
Indicated on display:
System in Basic Setting 30 ->
1 2 3 4
- Check oxygen sensor status for oxygen sensor behind catalytic converter in display field 2.
NOTE: "Oxygen sensor status" indicates status of oxygen sensor control and oxygen sensor.
Display Group 030:
Significance of 3 digit indication of display group 030
Checking oxygen sensor control behind catalytic converter
- Press -C- button.
Indicated on display:
System in Basic Setting Q
Input display group number XXX
- Press buttons -0-, -3- and -7- to select "display group number 037" and press -Q- button to confirm input.
Indicated on display:
System in Basic Setting 37 ->
1 2 3 4
- Check oxygen sensor voltage in display field 2.
- Check dwell time of oxygen sensor control before catalytic converter in display field 3 and diagnostic result in display field 4.
Display Group 037:
Evaluation display group 037
Continued
If specified value in display field 3 and 4 is not obtained:
- Road test vehicle in order to free oxygen sensor of possible residue and repeat test.
If specified value in display field 3 and 4 is not obtained, even after road test:
- Replace Heated Oxygen Sensor (HO2S) -G39-.
NOTE: If oxygen sensor voltage is OK and dwell time of oxygen sensor control behind catalytic converter is still above 1200 ms, even after a road test, this suggests aging of the oxygen sensor before catalytic converter.
Testing primary voltage
- Disconnect the 4-pin harness connector to the oxygen sensor behind catalytic converter (below a black plastic cover at underside of vehicle)
- Connect multimeter between terminal 3 and 4 for voltage measurement.
- Switch ignition on.
- Specification: 0.400 to 0.500 V
- Switch ignition off.
If specified value is not obtained:
- Test oxygen sensor wires.
If specified value is obtained:
- Replace oxygen sensor behind catalytic converter.
Testing oxygen sensor wires
- Disconnect the 4-pin harness connector to the oxygen sensor behind catalytic converter (below a black plastic cover at underside of vehicle).
- Connect VAG1598/31 test box at wiring harness to ECM, do not connect ECM. Wiring and Component Testing Using VAG1598/31 Test Box
- Check the following wire connections for open circuit.
Harness connector VAG1598/31 test box
Terminal Socket
3 68
4 69
- Repair open circuit if necessary.
- Check wires to each other for short circuit.
If wire connection is OK:
- Replace Engine Control Module (ECM).