Description

Audi S4 Fender Liner OEM Driverside

Audi B5 S4 fender liners are molded plastic inner panels that sit between the wheel and the outer fender, protecting the body, wiring, and engine bay from water, dirt, and debris while also helping manage airflow and cooling. They are shaped specifically for the B5 chassis and differ from A4 liners, particularly at the front where the S4 uses vented sections to aid brake and intercooler airflow.​

Purpose and Function

Fender liners form a barrier between the rotating wheel/tyre and the painted metal of the fender and inner structure.​
On the B5 S4 they help to:

• Prevent stone chips, mud, and road salt from blasting bare metal, reducing rust risk.​

• Shield wiring, washer hoses, ABS sensors, and side‑mount intercoolers (SMICs) from direct spray and impact.​

• Guide air through specific openings toward brakes and intercoolers rather than letting it escape randomly in the wheel well.​

Because the S4 runs larger brakes and a twin‑turbo 2.7T with side‑mount intercoolers, the liners are more than simple splash guards; they are part of the cooling and airflow package designed for the car’s performance envelope.​

Construction and Layout

Material and Basic Shape

OEM B5 S4 liners are vacuum‑formed or injection‑molded plastic (typically polyethylene or a similar thermoplastic) chosen because it is:

• Light, flexible, and resistant to cracking compared with metal.

• Non‑corrosive and able to flex with minor impacts rather than denting.

The inner surface follows the wheel arch contour along the fender lip and then wraps inward to cover the back of the bumper/fender area and the strut tower region.​
Fastening is by a mix of:

• Plastic push‑clips along the arch.

• Small self‑tapping screws or torx screws into the bumper and body mounting points.

Front S4‑Specific Details

Compared with A4, the front S4 liner is notable for:

• Integrated vents/cut‑outs to allow “straight through” airflow from the bumper openings through the wheel well.​

• Shaping that accommodates the S4 side‑mount intercoolers and associated ducting and washer bottle layout.​

These vented sections sit behind the fog‑light/intake grilles and allow pressurised air from the bumper to exit around the wheel, helping to:

• Cool front brakes.

• Improve intercooler efficiency by giving a lower‑pressure exit path.​

Rear liners are simpler, primarily covering the arch and lower quarter panel to keep dirt from being thrown into the rear bumper and inner arch cavities.​

Protection and Corrosion Control

One of the most important long‑term roles of the liners is corrosion prevention. Without them, the tyres throw water, salt, and gravel directly at:

• The fender lips and rolled seams.

• Inner sill and jack point areas.

• Rear bumper mounting points and metal brackets.

On older B5 chassis, cars that have run for years with cracked or missing liners often show:

• Rust on the rear arch lips and just ahead of the rear wheels.

• Corrosion at the lower front wing where dirt packs up behind the plastic under‑tray.

Keeping intact liners and clearing the mud that can build up behind their edges is a key rust‑prevention step on any B5, especially in wet or salted climates.​

Cooling and Airflow Considerations

The S4 liner vents are a subtle but important factory feature. Enthusiasts often note that:

• A4 liners are mostly closed; S4 units add perforations/vents that “provide straight through flow of air, for proper cooling.”​

• That airflow path is relevant for cars with upgraded brakes or higher‑output intercoolers, where wheel‑well heat can rise significantly.

Some owners:

• Swap S4 liners into A4s for better brake and SMIC cooling.​

• Trim or open up additional sections when fitting larger side‑mount intercoolers or brake cooling ducts, while keeping structural ribbing and attachment points.

Any modification must preserve:

• Enough strength so the liner doesn’t flap or crack.

• Coverage of wiring and paint to avoid stone damage.

Fitment, Differences, and Replacement

OEM Pieces and Sides

Each liner is side‑specific (left/right) and model‑specific (S4 vs A4). Sellers list them as:

• Driver side/front or passenger side/front fender liner.​

• Inner fender liner / plastic guard for “99–02 Audi A4 S4 B5.”​

B5 S4 liners bolt into the same general locations as A4 liners, but the venting, shape near the bumper, and cut‑outs for intercooler ducting differ, so using the correct part avoids interference with SMIC shrouds and ductwork.​

Common Damage and Symptoms

Typical issues on older B5 S4s:

• Cracked lower edges from kerb or ice contact.

• Broken mounting tabs/holes so the liner sags or rubs the tyre.

• Sections torn away due to tyre rub when running low ride heights or wide wheels/tyres.​

Symptoms include:

• Scraping noise while turning or over bumps.

• Visible gap between bumper and liner where you can see into the engine bay.

• More spray/dirt in the bay or inside the fender than normal.

Replacement Strategy

Replacement is straightforward:

• Remove the wheel.

• Undo screws and clips along the arch lip and under the bumper edge.

• Drop the old liner, clean accumulated dirt in cavities, then refit the new liner starting from the top and working around.

Aftermarket “gen 1” and similar replacement liners are offered as direct‑fit plastic reproductions, while used OEM liners are available from breakers and specialist Audi dismantlers.​

Interaction with Suspension, Steering, and Wheel Fitment

Because the liner sits very close to the tyre and strut, any change in:

• Wheel offset (ET),

• Tyre width/diameter, or

• Ride height/camber

can lead to rubbing on the liner at full lock or under compression. Enthusiasts running low ride heights sometimes:

• Heat‑gun and reshape small areas of the liner.

• Trim minor internal ribs or protrusions.

• Combine this with rolling the metal fender lips to create extra clearance.​

Care is needed to avoid cutting through to areas that protect wiring or to create openings that will channel dirt directly into the sill or behind the bumper.

Maintenance and Best Practices

To keep B5 S4 fender liners doing their job:

• Inspect annually for cracks, missing fasteners, and gaps around the bumper and arch.

• Remove wheels occasionally to wash out the mud and debris trapped behind the liner edges and at the lower corners.

• Replace missing clips/screws; a loose liner will eventually fatigue and crack.

• After any front‑end work (brakes, suspension, intercoolers), confirm the liner fits back correctly and doesn’t contact hoses or harnesses.

Owners doing big‑brake or intercooler upgrades often plan liner modifications at the same time to preserve both airflow and protection, instead of running without liners, which greatly accelerates corrosion and contamination.

Summary

Audi B5 S4 fender liners are more than simple plastic covers: they are shaped, vented components that protect structure and wiring from the harsh environment inside the wheel well while also helping move air past brakes and intercoolers. Using intact, correctly fitted S4‑specific liners—and maintaining them—pays off in better cooling, less rust, and a cleaner, more durable chassis.