Audi Workshop Service and Repair Manuals

Note:

The lambda control function after (i.e. downstream of) the catalytic converter takes precedence over the lambda control before the catalytic converterand serves as a corrective regulation.

→ It corrects slight variations in the fuel mixture (e.g. an enriched mixture) originating from the lambda probe before the catalytic converter. It does so by holding the lambda control before the catalytic converter for a specified period of time (delay period) at the richest or leanest state. If the delay period is on the positive side (50 ms in the example), then the mixture will be enriched. Conversely, if the delay period is on the negative side (-50 ms in the example), the mixture will be made leaner.

→ The 4-pin connector -1- (brown) for the lambda probe (after the catalytic converter) -G130 and lambda probe heating -Z30 is located underneath the coolant expansion tank.

Special tools and workshop equipment required

• ◆ V.A.G 1526 A
• ◆ V.A.G 1594 A
• ◆ V.A.G 1598/31
• ◆ V.A.G 1788/10
• ◆ VAS 5051 with VAS 5051/1

or

• ◆ V.A.G 1551 with V.A.G 1551/3 A

Requirements for test:

• ● Take the car for a test drive and do not erase the fault memory.
• ● Coolant temperature at least 80 °C

Test sequence

• ‒ Connect vehicle diagnostic, testing and information system VAS 5051 (or fault reader V.A.G 1551) and select engine electronics control unit by entering address word "01" ( .)
  When doing this the engine must be idling.

• ‒ Enter "04" to select "Basic setting" and confirm with Q key.

Note:

During the basic setting the activated charcoal filter solenoid valve (ACF valve -N80) is closed and the air conditioner compressor is switched off.

• ‒ Enter "034" for Display Group 034 and confirm entry with Q key.

• ‒ Set an engine speed of 2800...3200 rpm using speed regulator V.A.G 1788/10:
• ‒ Continue the test as soon as an exhaust temperature of more than 350 °C is reached in display zone 2.

Note:

This process can take a few minutes.

• ‒ Press C key.
• ‒ Set an engine speed of 1800...2200 rpm using speed regulator V.A.G 1788/10:

• ‒ Enter "030" to select Display Group 030 and confirm entry with Q key.

• ‒ Check status of lambda probe (after catalytic converter) in display zone 2.

Note:

The "lambda probe status" display indicates the status of the lambda control and lambda probes.

Relevance of 3-figure display in Display Group 030

Notes:

• ◆ The lambda control of lambda probe (after the catalytic converter; B1-P2) is not active when the engine is not running under load.
• ◆ The first digit of the three-digit display (heating) fluctuates between 0 and 1.

Testing lambda control (after catalytic converter)

• ‒ Press C key.

• ‒ Enter "037" for Display Group 037 and confirm entry with Q key.

• ‒ Check lambda probe voltage in display zone 2:
• ‒ Check the delay period of the lambda control (before the catalytic converter) in display zone 3 and the result of the diagnosis in display zone 4.

Note on display zone 3:

The lambda control after the catalytic converter (downstream) takes precedence over the lambda control before the catalytic converter (upstream). The lambda control after the catalytic converter makes corrections in the mixture control process to compensate for slight variations in the fuel mixture (e.g. an enriched mixture) originating from the lambda probe before the catalytic converter. It does so by holding the lambda control before the catalytic converter for a specified period of time (delay period) at the richest or leanest state. If the delay period is on the positive side (50 ms in the example), then the mixture will be enriched. Conversely, if the delay period is on the negative side (50 ms in the example), the mixture will be made leaner.

Reading Display Group 037

Further notes

If the readout in display zone 3 or 4 does not match the specification:

• ‒ Take car for a test drive to free lambda probe of any deposits, then repeat the test.

If the readout in display zone 3 or 4 still does not match the specification even after the test drive:

• ‒ Replace the lambda probe (before the catalytic converter) -G39.

Note:

If the lambda probe voltage is OK and the delay period for the lambda control after the catalytic converter is still above 1200 ms even after a test drive, this points to excessive ageing of the lambda probe before the catalytic converter.

→ Testing basic voltage

• ‒ Unplug 4-pin connector -1- (brown) for lambda probe (after catalytic converter) -G130.

• ‒ → Connect multimeter (voltage measurement range) between contacts 3 and 4.
• ‒ Switch ignition on.
• ‒ Specification: 0.400 ... 0.500 V
• ‒ Switch off ignition.

If the specification is not obtained:

• ‒ Test lambda probe wiring.

If the specification is obtained:

• ‒ Replace the lambda probe (after catalytic converter) -G130.

Testing lambda probe wiring

• ‒ Unplug 4-pin connector -1- (brown) for lambda probe (after catalytic converter) -G130.
• ‒ Connect test box V.A.G 1598/31 to wiring harness for engine control unit. Do not connect to the engine control unit itself.
  => Page 24-20.

• ‒ → Test the following wiring connections for open circuits:

• ‒ Rectify any open circuits.
• ‒ Test for short circuits between the wires.

If the wiring is OK:

• ‒ Fit a new engine control unit => Page 24-24.