Hong Hong Diagnostics | 2012 Focus Scirocco Engine Occasionally Fails to Start

Failure phenomenon

A 2012 Volkswagen Scirocco with a CCZ engine and approximately 96,000 kilometers.The owner reported that the vehicle occasionally failed to start the engine.

Fault Diagnostics

After connecting the car, we found that when the brake pedal is pressed and the start button is pressed, the engine will not start. The dashboard sometimes fails to light up and the starter motor does not respond. After resting for a while or disconnecting and reconnecting the negative electrode of the battery, the engine will start normally again. However, during operation, theMultiple warning lights on the instrument panel will illuminate abnormally and the tachometer will not display.

Tests with the Troubleshooter revealed that the Communication fault codes beginning with “U” are stored in several control units, with "Loss of communication with instrument board" being the most frequent code.

Repeated test drives have confirmed that if the instrument light is illuminated abnormally while the engine is running.The diagnostic instrument cannot communicate with the instrument panel at this time.The control unit can be connected to other control units normally. Summarizing the above phenomena, the preliminary judgment isAbnormalities in the dashboard body or its CAN bus wiring。

After checking the circuit diagram, we know that there are five CAN buses in the gateway (J533), which are Drive CAN, Comfort CAN, Infotainment CAN, Diagnostic CAN, and Dashboard CAN.

Fig. 1 Circuit of 故J533

Locate the gateway in the driver's seat footwell (Fig. 2), using.Acer Pico Car OscilloscopeMeasurements are made from gateway connector T20 terminals 8 (CAN L) and 18 (CAN H).The result shows that the CAN signal waveform is abnormally dense when the fault occurs (Fig. 3).

Fig. 2 Location of the gateway

Fig. 3 Signal waveforms of the combined instrument CAN bus become dense

At this point, unplug the meter board connector T32, the waveform change is obvious - the dense signal disappears, leaving only two straight lines of about 2.2V (Figure 4).This means that the abnormal signals are coming from the instrument panel and the gateway is not sending any signals to the instrument panel CAN bus. It can be inferred that the gateway (J533) has an internal failure, causing an occasional loss of communication with the instrument panel, which in turn causes the engine to fail to start.

Fig. 4 Signal waveform of the CAN bus of the pass-through combination meter becomes two straight lines.

fault resolution

After replacing the communication chip inside the gateway (Fig. 5, Model No. ATIC83 E2 32330)After a number of test drives, the problem has not recurred and has been successfully eliminated.

Fig. 5 Location of Communication Chip

Fault Summary

In the case of the Fuchs Scirocco, which occasionally failed to start, it was finally concluded thatThe fault is caused by the communication chip inside the gateway.

ThroughHonghong pico automotive oscilloscope的高解析量測，工程師能即時掌握 CAN 匯流排通訊狀況，準確定位問題來源，成功排除故障。 此次的故障案例分享，可以證明精密量測工具在現代車輛診斷中的重要性，也展現出The professional value of data-driven maintenance and accurate diagnosis.