Checking Control Function With Scan Tool

- Select "Reading measuring value block" function 08 on VAG 1551 Scan Tool (ST) and input display group 01. Refer to On-Board Diagnostics. Testing and Inspection
- Observe display value in channel 3.
^ Must be: 0.96 - 1.04.
^ Display must not remain at 1.0 (average value); value must fluctuate.

If specified value is NOT obtained:
- Check for the following malfunctions:
^ Fuel system pressure too high or too low.
^ Leak in intake air system between Mass Air Flow (MAF) sensor.
^ Leak in exhaust system between engine and Heated Oxygen Sensor (HO2S) probe.

Note:
A display value OVER 1.04 indicates that the HO2S has detected a lean condition, and to compensate, has enriched the control system.

The following can produce a lean condition:
^ Low fuel system pressure.
^ Auxiliary air intake in intake area.
^ Air leak forward of HO2S.

A display value UNDER 0.96 indicates that the HO2S sensor has detected enrichment and to compensate, has created a lean condition.
The following can produce enrichment:
^ Excessive fuel system pressure.
^ Air leak in system past turbocharger.

A display that remains fixed at the average value 1.00 indicates the following:
^ Electrical malfunction in HO2S control system.
^ Open circuit.
^ Short circuit.
^ Sensor probe malfunctioning.
^ Control module malfunctioning.
^ Insufficient heating of HO2S probe.