Audi A6

The timing chain of the 2.7 TDI V6 can stretch prematurely, especially with short-trip use and infrequent oil changes. Rattling at cold start is the warning sign. Full chain kit replacement costs €1,500–3,500.

Symptoms: Rattling or clattering right after cold start, engine management faults in memory, engine stalling in worst case

The EGR system on the 2.7 TDI V6 sits in the poorly accessible V-space between cylinder banks. Valve and cooler carbon up; the cooler can develop leaks and lose coolant.

Symptoms: Juddering and power loss, coolant loss without visible external leak, engine warning light with EGR fault code

The piezo injectors in the 2.7 TDI reach the end of their service life at very high mileages. Individual replacement is possible, but all 6 are often changed at the same time.

Symptoms: Rough running, elevated fuel consumption, smoke clouds at cold start, fault code for individual cylinders

Shutting off immediately after hard driving and insufficient oil warm-up at cold start accelerate turbocharger bearing wear. A failure leads to oil leaks and potentially total engine damage.

Symptoms: Whistling or grinding noise from the turbo area, bluish smoke, power loss, oil loss

The oil-to-water heat exchanger can develop leaks at high mileages, mixing coolant into the engine oil or passing oil into the cooling system. A relatively inexpensive replacement prevents engine damage.

Symptoms: Milky residue on the oil filler cap, coolant level drops without visible loss, oil looks milky-brown

The continuously variable Multitronic gearbox (01J/0AW) is considered the most serious weakness. Judder, rev fluctuations and complete failure. Early build years up to 2007 are particularly badly affected.

The A6 Allroad with adaptive air suspension frequently suffers from defective air spring bellows and overloaded compressors. Leaking bellows force the compressor to run continuously until it fails.

Blocked drain channels under the battery and sunroof allow water to enter the cabin. The comfort control module in the driver's footwell is damaged, leading to MMI total failure.

The Allroad with air suspension suffers from defective compressors and leaking air bellows. A leaking air bellows causes the compressor to run continuously until it overheats.

The Multitronic CVT tends to judder when pulling away, rpm fluctuations at partial load and, in the worst case, complete failure. All front-wheel-drive models are affected.

The high vehicle weight (up to two tonnes) accelerates wear of control arms, joints and anti-roll bar bushes. From around 150,000 km, extensive suspension work is virtually unavoidable.

Older A6 C6 examples show corrosion starting at the sills, door sill lower edges and tailgate. Where paint is damaged in the underbody area, corrosion progresses rapidly.

The cast-aluminium front control arms are prone to rubber bush wear. The long steel bolts in the wheel carrier corrode particularly badly and become extremely difficult to remove.

Pre-facelift vehicles (up to 2001) in particular show rust at the front wheel arches, door sill lower edges, and on the Avant at the roof edge above the windscreen.

Blocked drain holes under the battery lead to water ingress into the interior and can damage control units. Leaking cabin air filter housing seals exacerbate the problem.

The shaft seal between gearbox and propshaft on the A6 C5 eventually loses its seal. Gearbox oil drips onto the catalytic converter and burns off there. Often only diagnosed by smell or visual inspection from underneath.