If there is a complaint, a pin-pointed troubleshooting must be carried out:

First switch on the ignition and observe lighting up of the warning lights. TABLE OF WARNING LIGHT FUNCTIONS enables an initial determination.
Determine test preconditions. See TEST REQUIREMENTS .
Connect Scan tool V.A.G 1551 and select "Brake electronics" with address word 03. See VAG1551 SCAN TOOL (ST), CONNECTING AND ENTERING ADDRESS WORD .
Check control module version (see CHECK CONTROL MODULE VERSION ) and press --> button.
Check DTC memory (see CHECK DTC MEMORY ). If a DTC is stored: Make a note of the DTC and erase DTC memory (see ERASE DTC MEMORY ).
Complete data transfer with the function "End output". See CAN-BUS, FUNCTION .
Repair malfunction according to DTC table (see CHECK DTC MEMORY ). Reselect "Brake Electronics" with address word 03. Check and erase DTC memory.
Complete data transfer with the function "End output". Disconnect V.A.G 1551.
Make sure that all connectors are connected correctly. Perform road test, see ROAD TEST . The warning lights must not light up during the road test. If the warning lights light up during the road test the troubleshooting must be performed again.

NOTE: If the connector is disconnected from the control module during the drive there is danger of setting a false DTC entry in the DTC memory of another control module. The mechanical and hydraulic functions of the brake system remain effective. However, due to the failure of the ABS/EDL system the brake pressure regulator will also no longer work, which could endanger the safety of the vehicle when driving.

CAUTION: On vehicles equipped with Audi Telematics by OnStar(R), switch-off the emergency (back-up) battery for the Telematic/Telephone Control Module prior to disconnecting the vehicle battery. EXAMPLE: SWITCH OFF EMERGENCY (BACK-UP) BATTERY . Switch-on the emergency (back-up) battery after reconnecting the vehicle battery, see .EXAMPLE: SWITCH ON EMERGENCY (BACK-UP) BATTERY .

Switch off the ignition before removing and installing components. If the vehicle is equipped with a coded radio, obtain the security code before disconnecting the battery Ground (GND).
If a component is indicated as being faulty, only replace it if you are sure that the wiring for that component is in proper condition. The wiring must be able to transmit the necessary signals without interference or resistance. Check the wiring for:
- Corrosion and loose connections in the plug connectors and Ground (GND) connections. If any of the connector terminals are bent, broken or corroded, use the wiring repair kit VAS 1978 to install new terminals
- Open circuits
- Short circuits to Ground (GND) or to positive
See appropriate System Wiring Diagrams article.

NOTE: The instructions supplied with wiring repair kit VAS 1978 normally do not permit repairs to the wiring of the ABS and its associated systems. However, this only applies to the screened wiring in the systems.
If control module has to be replaced make sure the correct version is installed. Check the control module index first. This appears on the display of the Scan tool V.A.G 1551 together with the encoding once communications have been established between control module and Scan tool, or it can be called up by selecting function 01 "Checking control module version". See CHECK CONTROL MODULE VERSION .
If the hydraulic control module has to be replaced make sure the correct version is installed. Check the control module version first. This appears on the display of the Scan tool V.A.G 1551 once communications have been established between control module and Scan tool, or it can be called up by selecting function 01 "Checking control module version". See CHECK CONTROL MODULE VERSION . It is also possible to identify the equipment installed on the vehicle from a combination of letters on the vehicle data sticker. The sticker is in the spare wheel well.
1AC: Anti-lock brake system

1AH: Anti-lock brake system with electronic differential lock

1AJ: Anti-lock brake system with electronic differential lock and traction control system (ASR)

1AT: Anti-lock brake system with electronic differential lock, traction control system (ASR) and electronic stability program (ESP)
Absolute cleanliness is required when working on the hydraulic unit. Carefully clean the hydraulic connections and adjacent parts. Never use aggressive cleaning agents such as brake cleaner, gasoline, thinner or products containing mineral oil, oils and greases. Do not disconnect hydraulic lines before they have been cleaned.
Do not remove bolts securing individual components of the hydraulic unit. This does not apply to the bolt securing the cover. The cover must be detached in order to replace either of the two relays for the return pump or the solenoid valves.
Observe the relevant information when handling brake fluid.
See BLEEDING BRAKE SYSTEM .
Place removed parts on a clean surface and cover. Only use lint-free cloths.
Only use genuine wrapped parts. Only remove them from their packaging immediately prior to installation.
Bleed the brake system if connections in the system have to be opened.
See BLEEDING BRAKE SYSTEM .
Welding work with an electric welding unit can affect the ABS, ABS/EDL and ABS/EDL/ASR system.