Audi A4 B9 (2015-2024) Common Problems: What to Check Before You Buy

The B9 A4, sold as a saloon, Avant and allroad from late 2015 until the car was replaced in the mid-2020s, is a big step on from the B8 in the areas that used to cost owners money. The oil-drinking four-cylinder petrol and the Multitronic CVT are both gone, and the electrics are better sealed than the previous car's. It is not fault-free, though, and the faults it does have are specific enough to check for. The headline items are the combined coolant pump and thermostat module on the 2.0 TFSI petrol, the AdBlue and NOx sensor side of the diesel emissions system, the seven-speed S tronic dual-clutch gearbox at low speed, MMI screens that blank and restart, and hydraulic front suspension bushes that knock. Almost everything below depends on which engine and which side of the 2019 facelift you are looking at, so the first job is always to establish exactly what the car in front of you is. Our separate guide to the B8 A4 covers the previous generation, which is a very different set of concerns.

Which B9 is it: engines, facelift and timing drive

Get the specification straight before you go hunting for faults. The volume engines are the 2.0 TFSI petrol, from the EA888 family, in a lower-output version that runs a lean, long-expansion combustion cycle and a higher-output version, and the 2.0 TDI diesel from the EA288 family in a couple of power levels. A smaller 1.4 TFSI petrol was sold in some European markets early on. Above those sat a 3.0 V6 TDI in higher-spec cars, and V6 petrols: the S4 uses a 3.0 TFSI V6 that is turbocharged in this generation, not supercharged as the B8's was, and the RS4 Avant uses a 2.9 twin-turbo V6.

Two changes often get run together here, and it is worth separating them. The badging change came first and has nothing to do with the facelift: Audi announced its two-digit power designations in 2017 and worked them across the range over the following months, so late pre-facelift B9s already wear 35, 40 or 45 TFSI and 30, 35 or 40 TDI badges. A number on the boot lid therefore tells you an output band rather than an engine capacity, and it does not tell you which side of the facelift the car sits on. What the 2019 facelift itself brought on the four-cylinder cars was a 12-volt mild hybrid system, fitted to the four-cylinder diesels as well as the petrols rather than to the petrols alone. Emissions hardware and software were revised through the production run too, which is why the diesel complaints below cluster on the earlier cars.

Timing drive matters and differs. The 2.0 TDI runs a toothed cambelt. The interval is long and Audi's own schedule is the number to use, not a figure off a forum, but it is still a scheduled job on an interference engine, and it is normally done with the water pump, tensioner and rollers while the covers are off. The 2.0 TFSI and the V6s run chains, which are not a service item but are expensive to reach if they ever need attention.

The 2.0 TFSI: the coolant module, the breather and carbon

The best-known B9 petrol fault is the combined coolant pump and thermostat module. It is a plastic assembly with an electrically controlled thermostat, and it seeps rather than bursts. That means the symptom is usually a low coolant warning, a header tank that keeps dropping with no puddle on the drive, sometimes a crusty dried residue on the block, poor or inconsistent cabin heating, or a temperature gauge that behaves oddly. Specialists generally replace the module as a unit rather than trying to fix one part of it. It is worth having a dealer check the VIN for any outstanding recalls or service actions, but budget for the repair rather than assuming any of it will be covered.

Treat a coolant warning as urgent. If the temperature gauge climbs into the red or you get an overheating message, stop, switch off and let it cool rather than pressing on; a modern aluminium engine does not take much overheating before the damage is expensive and permanent.

The crankcase breather, or PCV, is the other genuine wear item on this engine family. A split diaphragm gives a hissing or whistling noise, a lumpy idle, oil pushed past seals and lean running codes, and it is a straightforward thing to listen for with the bonnet up. Carbon on the back of the intake valves deserves a mention but not a headline. These are direct-injection engines and deposits do collect over time, showing as rough cold running and misfires, but the B9's 2.0 TFSI also injects into the inlet port at part load, and that keeps the valves far cleaner than the B8's purely direct-injected engine managed. Treat it as something to check on a high-mileage car rather than something to expect. The heavy oil consumption the B8's four-cylinder was notorious for is likewise not a B9 story, though you should still check the level yourself and ask how often it needs topping up. Be careful with cold-start rattle for the same reason: the timing chain tensioner failures Audi four-cylinders are infamous for belong to the earlier engines rather than this one, so a rattle in the first second or two is not a B9 expectation, but if you hear one it is still worth having investigated rather than living with.

The diesels: AdBlue, NOx sensors, EGR and the DPF

The B9 diesels use selective catalytic reduction, which injects AdBlue into the exhaust, and that system is the single most common source of diesel complaints on this car, particularly on the earlier build years. The usual failures are the NOx sensors, the AdBlue dosing or metering unit, and the tank heater that keeps the fluid liquid in cold weather, which is why a fault that has been quiet all summer can appear on the first properly cold morning. Take the warnings seriously: the emissions system escalates from a message to a countdown, and if it is not fixed the car will eventually refuse to restart. That is designed behaviour, not a breakdown, but it will strand you at the worst moment.

EGR is the other diesel theme. The valve and cooler soot up and stick, giving poor running and an engine light, and a cooler that has cracked internally loses coolant into the exhaust stream, which shows up as steady coolant loss with nothing visible on the ground and often a sweet-smelling exhaust. On the V6 diesel the cooler sits in the vee of the engine under the inlet manifolds, so the part is not the problem, the labour to reach it is. The particulate filter is happiest on a car that does regular long runs; a short-journey life gives DPF warnings, limp mode and an oil level that creeps up the dipstick as unburnt fuel dilutes the oil, which is itself a reason to stop and get it looked at rather than to keep driving and watch it rise.

Because emissions hardware and software on VAG diesels of this era has been through recalls and service actions, ask a dealer to run the VIN for anything outstanding and check the service history for what has already been done.

S tronic, quattro ultra and the rest of the drivetrain

Most automatic B9s use the seven-speed S tronic dual-clutch gearbox. It is a well-proven unit when it is looked after, and the two things that decide whether a given car is a good one are whether the scheduled fluid and filter service has been done and how it behaves at walking pace. Judder or shunting as you creep in traffic, a hesitation before drive takes up, a thump into reverse, harsh or flaring shifts, or a drop into a limited mode all point at clutch or mechatronic wear. Test it in a car park and in stop-start traffic, not just on an open road, and have the gearbox module scanned for stored faults before you buy. The higher-powered V6 and S models use a conventional torque-converter automatic instead, which is generally the less anxious choice and still wants its fluid service.

Four-cylinder quattro cars in this generation often use quattro with ultra technology, which runs front-wheel drive when it can and clutches the rear axle back in when it detects it is needed. It is normal for it to be imperceptible. What is not normal is a shudder or a hopping feeling in tight low-speed turns, a clunk as drive engages, or a whine or hum from under the middle of the car that changes with speed, all of which point at the coupling, the rear differential or their fluid. The S4 and RS4 use a permanently engaged mechanical centre differential instead.

On manual cars, listen for dual-mass flywheel rattle at idle that changes when you dip the clutch, and feel for judder on take-up.

MMI, batteries and electrical oddities

A very commonly reported B9 electrical complaint is the MMI screen, and the virtual cockpit with it, going black and restarting on its own, usually within the first few minutes of a drive and more often in cold weather. Audi released several software builds across the car's life, so a dealer update is the first thing to ask about, and Audi has also documented corrupt data from a paired phone as a trigger, which makes deleting and re-pairing phones a cheap thing to try first. A unit that reboots constantly or never wakes at all is more likely a hardware failure than a software one.

Battery behaviour is worth understanding on this car. It is full of modules that are supposed to go to sleep, and a single module that does not is a classic cause of a car that starts fine daily but is flat after a weekend. If a battery has been replaced, it needs to be registered and coded to the car's energy management, and a battery fitted without that is one that dies early and brings odd messages with it. Start-stop refusing to work is very often the battery's state of health or its monitoring sensor rather than a fault in the stop-start system itself.

Water ingress is much less of a signature fault than it was on the B8, but it costs nothing to lift the boot floor and check the spare wheel well for damp while you are there. Several control modules live in the back of this car and none of them tolerate sitting in water, so a wet boot is worth chasing to its source rather than drying out and forgetting.

Suspension, brakes and steering

The front suspension is a multi-link design and its known wear point is the fluid-filled hydraulic bush in the lower arm. When it splits and loses its fluid you get a knock or clonk over speed bumps and broken surfaces, a slightly vague, wandering feel at speed, and often uneven wear on the inner edge of a front tyre. Arms are usually replaced in sets and the car needs a proper four-wheel alignment afterwards; skipping the alignment is how people end up buying tyres twice. Anti-roll bar drop links rattle over small bumps and are a cheap fix.

The electronic parking brake is the usual VAG arrangement, with motors on the rear calipers. One cheap thing to rule out first is rear pads changed without putting the system into service mode, so if a parking brake warning appeared right after a brake job, look there before anything expensive. Signs of a genuine failure are a parking brake that will not release, one rear wheel binding, or one wheel noticeably hot or smelling hot after a normal drive. Cars that do gentle, short mileage also corrode their rear discs at the edges. Do not defer anything on this system: if a wheel is binding, if a wheel is hot or smells hot after an ordinary drive, if the pedal goes long or feels hard and unassisted, or if the car pulls to one side under braking, stop driving the car and have it inspected rather than finishing the week on it.

Steering is electromechanical, so there is no pump or fluid to check. If a steering fault appears the car stays steerable, but the assistance can drop out and the effort at the wheel goes up sharply, most obviously at parking and junction speeds. If that happens while you are driving, slow down, allow for the extra effort and pull over somewhere safe rather than pressing on, and do not set off on a long journey with an intermittent steering warning or clear it by restarting the car until the light stays off. Get the fault read. The same seriousness applies to the airbag light: an illuminated light means the system has recorded a fault and may not deploy as intended in a crash, so have it diagnosed rather than living with it, and note that in the UK it is an MOT failure as well.

What to check on a test drive or inspection

Arrange to see the car stone cold. A seller who has warmed it up before you arrive has removed the most informative minute of the whole inspection, on a petrol and a diesel alike. Take a torch and give yourself time before you drive anywhere.

Start it and listen to the first few seconds for chain rattle or diesel knock. With the bonnet up, check the coolant level in the header tank and look for dried residue around the coolant module and the front of the engine, check the oil level and condition, and listen at idle for the hiss of a failed breather. Then drive it in a mix that suits the car's faults: crawl in traffic and do a few tight, slow-speed manoeuvres, where an S tronic and a quattro ultra coupling both show their hands, then a long uphill pull in a high gear, which is where underboost and limp mode appear, then rough surfaces at low speed with the radio off to hear the suspension.

Before you commit, watch the whole dashboard through an ignition cycle so you know the airbag, ABS and steering lights all illuminate and then go out. Work the MMI for the length of the drive rather than glancing at it. Ask directly about the cambelt on a diesel, the S tronic fluid service, any AdBlue or emissions work, and whether the coolant module has been replaced on a petrol. Ask a dealer to check the VIN for outstanding recalls and service actions. And have every module scanned, not just the engine.

Fault codes on the B9, and what they usually point at

A stored code names a circuit or a plausibility check the car was unhappy with, not the broken part. On this generation the same code routinely has three or four possible causes, so read the code, then prove the cause with live data before anyone spends money. On the petrols, misfire codes are usually coils and plugs first, then injectors and, on a high-mileage car, carbon; a lean running code very often traces back to the breather diaphragm or an intake leak; a coolant-below-regulating-temperature code points at the thermostat, which on this engine means the same module that leaks. Cam and crank correlation codes on a chain engine should never simply be cleared. On the diesels, an underboost code is typically sticking turbo vanes, a split hose or a boost leak rather than a dead turbo, EGR flow codes go with a sooted valve or cooler, DPF codes go with a filter that has not been regenerating, and a low rail pressure fault should be diagnosed before the car is driven further rather than driven through. The SCR system has its own family of codes, with catalyst efficiency and NOx sensor faults usually appearing together, and on a car with steady coolant loss and no visible leak the EGR cooler is a prime suspect.

The other half of the picture is that the awkward B9 faults live outside the engine ECU. Generic OBD readers see engine emissions data only, while the gearbox, parking brake, airbag, comfort and infotainment modules report VAG's own codes in their own controllers, and a scatter of implausible-signal faults across several modules at once tells a very different story from one code in one place. That is the case for reading everything. VAGPULSE's free Android phone app connects over Bluetooth and does exactly that job for a fault hunt: fault codes, live data and a scan across modules. The Windows software and cable, at $249 as a single payment with the cable included, adds the deeper work such as live measuring blocks while a fault is happening, coding and adaptation, service functions such as putting the parking brake into service mode and registering a battery where the car supports them, and datalogging. Tuning is a separate matter and is deliberately conservative: VAGPULSE only builds and writes a map where it has hardware-verified map locations for that exact ECU software and refuses where it does not, and any write to an ECU carries risk, so back up first and read the warnings rather than assuming any given B9 is supported.

Common questions