2000-2016, 2018-2025 Audi S4 Problems & Issues

Certain 2000-2002 S4 vehicles have a non-azide driver airbag inflator that can absorb moisture and deploy improperly, reducing occupant protection in a crash.

Certain 2005-2009 S4 body styles contain passenger frontal-airbag inflators that can rupture after long-term humidity and temperature exposure, propelling metal fragments into the cabin.

On certain 2013-2015 S4 vehicles, an airbag-control algorithm may fail to command the front airbags after a side-airbag deployment followed by a second frontal impact.

On B9 S4 sedans, the cable connecting the front passenger seat heater to the Passenger Occupant Detection System (PODS) can develop a contact fault. The PODS software reads the fault as a system malfunction and switches the passenger front airbag OFF, so the airbag will not deploy in a crash even with an adult occupant in the seat. Audi recalled 208,332 vehicles for this on 10 November 2021 under NHTSA campaign 21V874000, internal code 74E3; the population includes 2018-2020 S4 Sedan along with A4 Sedan/Allroad, A5, S5 and RS5 variants. Owner notification letters went out 7 January 2022.

On the B5 S4's 2.7L twin-turbo V6 (APB/AGB), the turbocharger oil feed and return lines are too small in diameter to move enough oil through the turbo bearing housings. After a hot shutdown the residual oil bakes (cokes) inside the lines, progressively restricting flow until the turbo bearings are oil-starved and fail. This is a design issue specific to the 2000-2002 2.7T as fitted to the B5 S4 (and the contemporary A6/allroad); Audi went to larger-diameter lines on the 2003+ A6/allroad, but the S4 was discontinued after 2002 so NO B5 S4 ever got the revised parts from the factory. Failure typically shows as smoke, boost loss and oil consumption, and a failed turbo can dump oil and debris downstream.

Unlike every later S4, the B5's 2.7T 30V V6 uses a timing belt, and it is an interference design. A snapped belt, a seized tensioner/idler, or a water pump that fails and shreds the belt results in bent valves and, in some cases, cracked pistons - a total engine rebuild. The revised service interval for the 2.7T 30V is 75,000 miles, and because every B5 S4 is now 20+ years old, the age limit (roughly six years) matters more than the mileage: tensioners, the damper, the water pump and the associated seals and O-rings are all at or past end of service life. Cars bought used with unknown belt history are the common loss case.

The B6/B7 S4's 4.2L V8 (BBK/BHF) drives its timing chains from the BACK of the engine, against the firewall. What owners call "timing chain failure" is really the plastic chain guides, the chain tensioners and the two camshaft adjusters wearing out and disintegrating. Forum documentation shows plastic guide fragments sitting on the chains, with cases reported at roughly 100k-112k miles. Because the chain drive is at the rear, there is no way in from the front of the car: the engine (or at minimum the whole drivetrain assembly on a subframe drop) must come out. This is the single defining failure of this generation and is NOT shared with the B5 2.7T (belt-driven) or the B8/B9 V6s. Left alone, a snapped guide lets the chain jump timing and destroys valves and pistons on an interference engine.

The B8/B8.5 S4's Eaton TVS roots supercharger drives its rotor pack through a plastic coupler disc between the drive snout and the driven rotor. Over time the drive holes in that plastic coupler elongate and wear, and the slop shows up as a distinct rattling / marbles-in-a-can noise from the front of the engine at idle and low rpm, usually going away as revs rise. This is a wear item specific to the supercharged 3.0 TFSI (B8/B8.5) - it does not exist on the B9's turbocharged EA839, the B6/B7 V8, or the B5 2.7T. It is not immediately dangerous, but ignoring it lets the wear load the supercharger's bearings and shaft seals, turning a coupler job into a full blower rebuild.

The supercharged 3.0 TFSI's crankcase ventilation module (PCV / air-oil separator) uses an internal diaphragm that hardens and tears. When it does, the engine pulls unmetered air through the crankcase, gives up its ability to control crankcase pressure, and starts drawing oil into the intake. Owners typically see it between roughly 60,000 and 80,000 miles. Symptoms overlap with several other faults, which is why it is so often misdiagnosed - the tell is a rough or hunting idle combined with rising oil consumption and lean/crankcase-ventilation fault codes. Excess crankcase pressure also pushes oil past the rear main and cam seals, so a neglected PCV can create leaks that look like a much bigger problem.

The B9 S4's turbocharged EA839 3.0T uses a water pump whose internal seals fail, and the failure mode is unusual: coolant does not only weep externally, it is drawn INTO the engine's vacuum circuit and other places coolant should never reach, contaminating vacuum lines and components. Because much of the leak is internal, an owner can lose coolant with nothing visible on the ground, and the first warning may be a low-coolant message or an overheat. Owners and specialists describe it as a when-not-if item on this engine family. Audi has been hit with a class action alleging the water pumps are defective in a range of 2.9T and 3.0T EA839 models, with plaintiffs claiming the automaker knew of the defect since at least 2018; the S4 is not among the models named in the published vehicle list. Note this is a DIFFERENT part and failure mode from the B8 supercharged 3.0T's plastic water pump / thermostat housing.

The S4's supercharged 3.0T V6 uses a plastic thermostat housing that sits in the engine valley beneath the supercharger, next to the PCV valve. The plastic stands up to repeated heat cycles less well than metal, and the housing is prone to cracking and leaking coolant. The plastic tabs that hold the thermostat can also break, which can leave the thermostat stuck closed and overheat the engine. Because the housing is buried under the supercharger, a housing leak usually shows up as coolant pooling in the valley or a sweet smell after a drive, often with no temperature warning. Leaks are therefore hard to spot until a lot of coolant has been lost. The thermostat itself is a separate weak point. It can stick open, which makes warm-up slow and can set P0128. It can also stick closed, which makes the engine heat up fast. Audi bulletin 01 15 24 covers a "Coolant System: Fault Turn Off Engine" warning (DTC P1B1C) on 2012-2015 3.0T models. Audi says the cause is either a thermostat defect on 2012-2013 cars or software that shows the warning when nothing is wrong. Specialist shops suggest planning for the thermostat around 60,000-80,000 miles.

Lock nuts from particular production batches used on the eccentric bolt at the rear-axle trailing arm can crack from stress corrosion. If the nut breaks, the eccentric bolt can move and the rear-axle wheel alignment changes - and at higher speed the rear axle can suddenly and uncontrollably steer in the wrong direction, a loss-of-control risk. Audi issued NHTSA campaign 21V295000 (internal code 42L1) in 2021 covering a broad Audi range that includes the 2020-2021 S4 Sedan. Because the symptom is a gradual alignment change, some owners noticed it as steering-wheel offset or a pull long before understanding the cause. A follow-on campaign (22V034000) addressed vehicles where the required alignment check was not performed after an earlier repair.

The B5 S4 uses a multi-link front suspension with four control arms per side, and both the rubber bushings and the integrated ball joints are known short-life items - a problem noted even on relatively fresh cars and now universal given the fleet's age. The rubber compound hardens, cracks and separates with age and heat exposure regardless of mileage, so a low-mileage garage-kept B5 is no safer a bet than a driver. The result is clunking over bumps, imprecise or wandering steering, and uneven tire wear; badly deteriorated arms are also a safety concern because the ball joints are non-serviceable and integral to the arm. CV boots are a commonly co-occurring wear item on the same cars.

S4s ordered with the 7-speed S tronic dual-clutch use the 0B5 (DL501) gearbox, whose mechatronic control unit - the valve body with its integrated solenoids, pressure sensors and TCU - is a documented weak point in high-torque applications like the S4. Failures start as harsh or jerky engagement and occasional gear-selection errors, then progress: limp home mode, sticking in gear, loss of half the gears, or no reverse. Root causes are solenoid regulation faults, clutch pressure-sensor faults, internal overheating, and speed-sensor faults. Because the S4's torque output loads the unit harder than the A4 applications sharing the same gearbox, S4 owners see it disproportionately. This is a transmission-specific issue - manual-transmission S4s are unaffected.

Manual-transmission B6 and B7 S4s use a dual-mass flywheel behind the 4.2 V8, and the internal damper springs fail. The failure is progressive and intermittent early on - owners describe shudder and vibration that appear for days, vanish, then come back and stay - before settling into a constant rattle at idle that quiets when the clutch pedal is depressed. On the B6 in particular, flywheels have been documented separating outright, which can do serious damage to the bellhousing area. The DMF is a wear item that must be replaced with the clutch, not resurfaced or reused, and catastrophic failures generally follow from running one past its service life.

Certain 2011-2012 S4 fuel-injection systems can leak at the fuel rails or corresponding seals, creating a fire risk near an ignition source.

Certain 2003-2004 S4 xenon headlamp reflectors can lose coating over time, reducing luminous transmittance and the driver’s field of view.

The secondary air injection (SAI) system pumps air into the exhaust ports on cold start to light off the catalysts, and it is a chronic check-engine-light source on these cars. Each bank has its own combi valve. Carbon causes the combi valve to stick open, which lets hot, moist exhaust gas flow backward and condense inside the pump housing, rusting the motor bearings - the source of the loud jet-engine or whining noise on cold start. A separate and commonly misdiagnosed variant produces the same P0491/P0492 insufficient-flow codes with NO pump noise at all: the air passages inside the cylinder heads are carboned shut, and replacing the pump will not fix it. Brittle hard vacuum lines to the combi valves are a third cause.