Audi A6

The Nikasil coating on the cylinder bores detaches, leading to piston slap, heavy oil consumption and power loss. Engine damage is inevitable; repair is often uneconomical.

Symptoms: Heavy oil consumption, knocking from the engine, power drop, engine failure in extreme cases

The 3.2 FSI AUK is known for piston slap — pistons or cylinder bores wear and produce knocking noises with increased oil consumption. Repair is rarely economical given the costs involved.

Symptoms: Knocking from the engine, increased oil consumption, sudden power loss, loud slapping under load

The timing chain on the 3.2 FSI can stretch and cause engine damage if it jumps. More common than on the 3.0 TDI. Rattling on cold start is the first warning sign.

Symptoms: Cold-start rattle, power loss, rough running, engine damage in the worst case

Direct injection without intake port wetting leads to carbon deposits on intake valves and in the combustion chamber. Coking causes rough running, poor cold-start behaviour and increased fuel consumption.

Symptoms: Judder on cold start, rough idle, increased fuel consumption, power loss

The AUK has four hydraulic camshaft adjusters. Faulty adjusters or their solenoid valves cause misfires, engine warning light and cold-start rattle. Replacement including timing chain often over 1,500 €.

Symptoms: Cold-start rattle, engine warning light, combustion misfires, fault code for camshaft phasing

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.