Audi A4 Mk5 (B9) Common Problems & Reliability

The Audi A4 Mk5 (B9), launched in late 2015, moved the A4 into a more sophisticated and technologically advanced space, designed to compete with the BMW 3 Series and Mercedes C-Class on refinement rather than outright sportiness. It is widely praised in the UK for its exceptionally quiet cabin, high-quality interior materials, and the introduction of the Virtual Cockpit. As a used buy, it is a staple of the executive car market, offering a mature and efficient driving experience that still feels current. Reliability is generally good, but for UK owners, the AdBlue and NOx sensor systems on diesel (TDI) models are the most frequent source of frustration. These systems are prone to sensor failure and crystallization of the AdBlue fluid, which can trigger a "no-start" countdown on the dashboard—a repair that can be costly if out of warranty. Mechanically, the EA888 petrol and EA288 diesel engines are robust, though the plastic water pump and thermostat housing are known leak points across the entire range. The 7-speed S tronic transmission is smooth but demands fluid changes every 38,000 miles to maintain longevity. When purchasing a B9 in the UK, prioritize cars with a documented S tronic service history and check for any "NOx sensor" or "AdBlue" related fault codes. LED daytime running lights (DRL) can occasionally fail, and because the units are sealed, a failure often requires an expensive complete headlight replacement. On the test drive, listen for any knocking from the front suspension, as the multi-link setup is sensitive to pothole damage. Finally, for those in major cities, verify the ULEZ status; while all petrols are compliant, only Euro 6 diesels avoid daily charges.