Audi Q5 - SCR system fault

In the above-mentioned vehicle type, the "SCR system fault - no engine start in XXX km" message may appears on the instrument cluster while no fault code is stored in the engine control unit. In many Q5 series models, the front NOx sensor is monitored exclusively using the exhaust gas aftertreatment control unit. The fault memory remains empty if merely the engine control unit is read out as part of diagnostics. If this fault applies but no definite faults can be identified in the periphery of the affected systems during troubleshooting, pay particular attention to the differential pressure sensor of the NOx sensor upstream of the SCR catalytic converter.

The sensor continuously records the nitrogen oxide concentration in the exhaust system and regularly transmits this information to the exhaust gas aftertreatment control unit. If internal electronic faults, thermal ageing or temporary communication interruptions result in implausible measured values, the control unit assesses the situation as an emission control fault and triggers the warning message.

The first step is to check the sensor's plug connection and the wiring harness for humidity, corrosion and line breaks. If there are no abnormalities here, go for a test drive and check the parameters of all SCR system control units for in-depth troubleshooting. Check NOx sensor live parameters for plausibility. If the value is constantly zero, i.e. an unrealistically low ppm value, this indicates a faulty sensor.

If the values during the test drive are implausible, replacing the NOx sensor according to the manufacturer's specifications may eliminate the issue. Use vehicle-specific data and the article number of the removed component as a reference to determine the correct spare part. Finally, always compare the OE article number to guarantee clear assignment.

After having replaced the NOx sensor, initialise the SCR system or calibrate it using a suitable diagnostic unit to ensure correct functionality.

Once the cause of the fault has been eliminated, delete the fault memory and then go for a test drive.

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