Audi A6 C6 (2004-2011): Common Problems and What to Check

The C6-generation A6 (2004-2011, facelifted in 2008) is a comfortable, solidly built saloon, Avant and allroad that can be perfectly dependable to live with, but what it costs you depends far more on which engine and gearbox it has than on the number on the odometer. The areas that come up again and again are the timing chains at the gearbox end of the V6 engines, the Multitronic CVT on front-wheel-drive automatics, worn front suspension arms, carbon and emissions clogging on the diesels, air suspension leaks on cars that have it, and water finding its way into the electrics. None of that means the car is a bad buy; it means the history file and a full all-module diagnostic scan are worth more here than on a simpler car. This guide goes through it system by system, and flags where the answer changes by engine or by year.

Timing: belt or chain, and why it decides your budget

Establish this first, because the two families behave completely differently. The four-cylinder engines - the 2.0 TFSI petrol and the 2.0 TDI diesel, in both the earlier pumpe-duse (PD) unit-injector form and the common-rail version that arrived with the 2008 facelift - drive their camshafts with a toothed belt. That is a plannable, routine job, but it has to have actually been done, and on age as well as mileage. A belt of unknown history is a genuine risk on these engines, because they are interference designs: a belt that lets go wrecks valves and usually pistons rather than simply stopping the car at the roadside.

The V6s run chains instead, and the chains sit at the flywheel end of the engine rather than the front. That applies to the 2.7 and 3.0 TDI diesels and to all the petrol V6s: the port-injected 2.4, the 2.8 and 3.2 FSI, and the 3.0 TFSI that replaced the 3.2 at the facelift. Note that in this generation the 3.0 TFSI is supercharged, not turbocharged, whatever the badge suggests. Chains are not a scheduled service item the way a belt is, and plenty of them cover a long life, but the chains, the guides and above all the tensioners do wear - and when they need attention the labour is severe, because the engine and gearbox have to be separated to reach the back of the engine.

The eight- and ten-cylinder cars are firmly specialist territory, and there is one detail here that is easy to get wrong. The 4.2 V8 fitted from launch is port-injected and belt-driven, with short chains linking its camshafts; the 4.2 FSI that replaced it in 2006 is direct-injected and chain-driven. Do not assume a big-engined C6 has no belt to worry about until you have established which of the two it actually has. The S6's naturally aspirated 5.2 V10 and the twin-turbocharged 5.0 V10 in the RS6 run chains. Buy any of these on documented history and a marque specialist's inspection, not on hope.

The warning sign on a chain engine is a rattle for a second or two from the back of the engine on a cold start, or a stored camshaft-to-crankshaft correlation fault. Do not keep driving a car that does either in the hope that it settles down. A worn tensioner or guide that lets the chain jump a tooth destroys the engine, and that brief cold-start rattle may be the only warning you get.

Petrol engines: fuel pump follower, carbon and coils

The 2.0 TFSI has one very characteristic weak point: the cam follower that sits between the camshaft and the high-pressure fuel pump wears through. It shows as hesitation, rough running under load, hard starting and low fuel-pressure faults. The follower itself is a small part, and inspecting it means removing the pump - but if it is left long enough it wears into the camshaft lobe underneath, and that is a different order of repair entirely. On a car with no evidence it has ever been looked at, treat checking it as a service item rather than a repair.

The direct-injection petrols - the 2.0 TFSI, the 2.8 and 3.2 FSI, the supercharged 3.0 TFSI and the 4.2 FSI - all suffer carbon build-up on the back of the intake valves, because no fuel is sprayed over them to keep them clean. It creeps up on you: a lumpy cold idle, a slight misfire, weaker throttle response, and misfire codes that wander between cylinders. The fix is mechanical cleaning of the ports, commonly by walnut blasting. Ignition coils and spark plugs are the other routine cause of misfires on these engines and are worth ruling out first, because they are far cheaper to address.

The 2.4 V6 is port-injected rather than direct-injected, so it sidesteps the intake-valve carbon problem, though it is thirsty for the performance it offers. It is still a chain engine, so everything said above about chains applies to it as much as to the FSI cars.

Diesels: flaps, EGR, DPF and injectors

The V6 TDIs (2.7 and 3.0) are strong, torquey engines with two well-known nuisances. The first is the intake manifold swirl flaps: soot builds up in the ports, the flaps and their linkage stick, and the actuator motor or the operating lever gives up. Typically you get a flap-position fault, sometimes reduced power or limp mode. The second, particularly talked about on the 2.7, is the variable-vane turbo actuator sticking with carbon, which shows as underboost, a flat spot, or the car dropping into limp mode after a hard pull. Vacuum hoses perish with age and cause very similar symptoms, so a leak test is worth doing before anything gets replaced.

On the 2.0 TDI, which version you have matters. The earlier PD engines are known for injector wear and for chafed or corroded injector wiring inside the rocker cover, giving a knock, rough running and cylinder-specific faults. They also drive the oil pump from the balance shaft module through a small hexagonal shaft, and that hex can round off - which loses oil pressure with very little warning and can destroy the engine. The drive was revised during the engine's production life and later units are stronger, so it is not a fault every car will suffer, but it is well known and worth asking about. Any history of an oil pressure warning on one of these needs a proper explanation, not a reset. The later common-rail cars trade those problems for more conventional common-rail wear: injectors, the high-pressure pump and rail-pressure faults.

Both diesel families share the emissions-hardware story. EGR valves and coolers clog, and cars fitted with a diesel particulate filter object to a life of short, cold journeys - repeated interrupted regenerations end in a blocked filter, rising soot load and eventually a forced regeneration or a filter clean. Ask how the car has been used. A motorway car and a two-mile-commute car in the same specification are not the same purchase.

Gearboxes and drivetrain

Front-wheel-drive C6s with an automatic use the Multitronic CVT, and it is the component buyers ask about most. Manuals were offered alongside it, so not every front-driven car has one. A healthy Multitronic is smooth and slightly odd-feeling; an unhappy one shudders or judders as you pull away from rest, particularly when hot or on an incline, hesitates before taking up drive, lets the revs flare under load, or drops into a limp mode where it feels stuck in one ratio. Fluid and filter changes at sensible intervals genuinely matter to these, and documented evidence of them is worth paying extra for. Audi revised the transmission during the C6's life, so later units have a somewhat better reputation, but condition and service history tell you far more than the build year. Test one properly rather than writing the type off - and equally, do not accept a judder as a quirk.

Quattro cars use the ZF six-speed tiptronic, a conventional torque-converter automatic with a much better reputation, but it is not maintenance-free either. A tired one slurs or flares between ratios, thumps into Drive or Reverse, or logs faults from the valve body and its electronics. Fresh fluid at a reasonable interval is cheap insurance compared with a rebuild. Six-speed manuals are straightforward; listen for dual-mass flywheel rattle at idle with the clutch up.

On quattro cars, feel for a vibration at speed that comes and goes with load - the propshaft centre bearing and universal joints are worth inspecting, along with the driveshaft boots, which split and throw grease around the arch.

Suspension, steering and brakes

The front suspension is a multi-link design with several aluminium arms per side, and the bushes and ball joints in them are the single most common source of complaints. Symptoms are a knock or clonk over broken surfaces and speed humps, a vague, wandering feel at motorway speed, wheel wobble felt through the steering under braking, and uneven wear on the inner edges of the front tyres. Individual arms can be replaced, but by the time one has gone the others are usually not far behind, so the job is normally quoted as a set per side plus an alignment afterwards. Track rod ends and anti-roll bar links wear alongside them, and worn steering and suspension joints are an MOT matter as well as a comfort one, so do not put this off.

Air suspension was standard on the allroad and optional on other models. Failures start with the air springs themselves leaking, and then the compressor burns out because it is working constantly to compensate. Warning signs are the car sitting low or nose-down after standing overnight, a slow or noisy rise, uneven ride height corner to corner, or a suspension warning message. Catching a small leak early is far cheaper than replacing a leak plus a compressor.

The C6 uses an electromechanical parking brake, with an electric motor on each rear caliper. Those motors and their wiring can fail, giving a parking brake warning, a brake that will not release properly, or a grinding noise as it applies - none of which should be lived with on a braking system. It also means the rear pads cannot be changed by winding the pistons back: the system has to be put into service mode with diagnostic software first and reset afterwards, and cycling the parking brake while the pads or calipers are off is a known way to damage the mechanism. Factor that in if you do your own work, and scan the car for existing parking brake faults before starting. Steering on this generation is hydraulically assisted, so check for power steering fluid leaks from the pump and hoses, and listen for a whine on full lock.

Electrics, MMI and water ingress

Water is the root cause of a surprising share of C6 electrical mysteries. The drains at the base of the windscreen scuttle and, on cars with one, the sunroof drains block with leaf debris, and the water that cannot escape ends up in the footwells and under the carpet. Because connectors and control modules live low down in the car, the resulting faults look nothing like a leak: intermittent central locking, windows behaving oddly, unexplained warning lights, or a car that will not recognise its key. If the carpets are damp, fix the drains before anyone starts changing modules. Check the boot and spare wheel well too, especially on Avants.

The infotainment system is worth understanding before you panic. The head unit, amplifier, tuner, telephone and changer sit on a fibre-optic ring, so a single failed unit or a disturbed optical connector anywhere in that loop can take the whole system down at once - no sound, an unresponsive MMI, no telephone. That is a fault-finding exercise to identify which node has broken the ring, not automatically a new screen. MMI control units do also fail or freeze in their own right, and a car that goes flat after standing a few days often has a module that is not going to sleep.

Two more to note. An electronic steering column lock that fails leaves you with a no-start and a steering lock message, which is alarming but a known and fixable fault. And an airbag warning light needs treating as what it is: while that fault is stored, part of the restraint system may be switched off and may not work as intended in a crash, so have the stored code read and the cause repaired rather than simply cleared. On these cars the cause is commonly wiring under a front seat - the belt tensioner or occupancy sensor loom chafing where it passes the seat runners - or the clock spring in the steering column, and both are repairable. Finally, clicking noises behind the dash with one side of the car blowing the wrong temperature usually mean a climate control flap motor.

Fault codes you will actually see, and what they point at

A stored code tells you which circuit or function the car is unhappy about. It does not tell you whether the cause is a sensor, the wiring to it, or a real mechanical problem, and on this car a single underlying fault often lights up several modules at once. Read the freeze-frame data and the live values alongside the code before spending money. These are the ones that turn up most often on a C6:

• Camshaft/crankshaft correlation faults (P0016, P0017 and relatives) - on any of the chain-driven engines, petrol V6 or V6 TDI, this points at chain stretch, guides or tensioners. Treat as urgent and stop driving the car.
• P0299 underboost - on the TDIs, usually a sticking turbo vane actuator, a split boost hose or a vacuum leak, occasionally a clogged intake.
• P2015 intake manifold flap position - the classic swirl flap lever and actuator fault on the diesels.
• P0401 insufficient EGR flow, plus soot and ash load and regeneration faults from the particulate filter - clogging and short-journey use.
• P0087 fuel rail pressure too low - on common-rail diesels look at the high-pressure pump, a leaking injector return or a tired filter; on a 2.0 TFSI it is worth checking the cam follower and pump among the possible causes.
• Misfire codes (P0300 and the per-cylinder P030x) - coils and plugs first, then carbon build-up on the direct-injection engines, then injectors.
• Gearbox module faults about slip, pressure or limp mode on Multitronic cars - read the transmission control module directly, as a reader that only talks to the engine will miss them.
• Level control faults about system pressure or ride height - air suspension leaks or a struggling compressor.
• "No communication" faults appearing across several infotainment units at once - a broken fibre-optic ring rather than several dead modules.
• Airbag module faults naming a specific seat or belt circuit - most often wiring under the seat, but confirm rather than assume, because it is a restraint system.

What to check on a test drive or pre-purchase inspection

Insist on a genuinely cold start. Arrange it so the car has not been warmed up before you arrive - a seller who has already run the engine may not be hiding anything, but you have lost your best diagnostic moment. On the V6s, listen for a rattle in the first couple of seconds from the back of the engine. On diesels, listen for the clatter settling as oil pressure comes up, and watch the exhaust.

Then work through the car methodically:

• Establish exactly which engine the car has before anything else, because the belt-or-chain answer, and therefore the paperwork you need to see, changes with it.
• Multitronic cars: pull away gently uphill from rest, both cold and fully warm, and feel for shudder or hesitation. Then use full throttle and check the revs and road speed rise together.
• Tiptronic (quattro) cars: check engagement into Drive and Reverse is prompt and not a thump, and that kickdown is clean.
• Drive a rough road slowly with the windows down and listen for knocks from the front suspension; on a motorway, check for wander, vibration, and any wobble through the wheel under braking.
• Air-suspension cars: see the car after it has stood overnight, check it sits level, and cycle the height settings.
• Apply and release the electric parking brake several times, listening for both rear motors working, and scan for stored parking brake faults.
• Press the carpet in all four footwells with your hand, lift the boot floor and check the spare wheel well, and note any damp smell.
• Try every electrical function once: windows, mirrors, seats and memory, sunroof, all speakers, both sides of the climate control, all exterior lights and the headlight washers.
• Check the paperwork for a dated cambelt change on the belt-driven engines (the four-cylinders, and the early port-injected 4.2 V8), oil services to the correct specification on a diesel with a particulate filter, and any gearbox fluid changes.

Common questions