Audi S4 2.7 B5 (1997-2002) K03 Turbocharger

One Turbo Has Failed — On a 2.7T That Is a Question About Two

A B5 S4 does not have a turbocharger. It has a pair, one to each bank of the V6, and that changes what a failure means: the unit in your hands is the one that gave up first, not the only one that has been working.

Both have turned the same hours behind the same engine, fed by the same oil through feed pipes of the same age, and both have heat-soaked every time it was shut down hot. Wear on a journal-bearing turbocharger follows duty and oil condition rather than luck, so whatever aged the unit you removed has been working on the other at the same rate.

That is not an argument for replacing both blind. It is an argument for measuring the second one while it is accessible — shaft play, oil traces at the turbine end and free movement at the wastegate take minutes to check on a turbo already off the car, and cannot honestly be judged on one still bolted in place.

Removing and refitting a 2.7T turbocharger is a far bigger job than the bench work that follows, and that effort is the same whether one unit comes off or both.

What Wears Out on a 2.7T K03, and What the Rebuild Renews

These are wastegated turbochargers: no vane mechanism inside, no electronic actuator on the side. The faults are rotating-assembly and exhaust-side ones, not the carbon-and-calibration problems of a modern variable-geometry diesel.

• Journal bearings and shaft play. Clearance opens up with hours and oil condition. Long before anything breaks you get a whine that rises and falls with boost, and wheel tips that eventually touch their housings.
• Oil seals. A worn seal lets oil past into the compressor or turbine side, and on a twin-turbo engine it usually shows on one side only: oil misting in one bank’s pipework, blue smoke, oil being used with nothing leaking.
• The wastegate mechanism. The exhaust-side pivot and the actuator rod corrode and stiffen with heat and age. A wastegate that no longer holds position gives boost that overshoots or never arrives, and it reads as a code rather than a noise — P0299 for underboost, P0234 for overboost.
• Oil feed and return. Feed pipes carbon up internally and returns restrict. A bearing that is starved or cooked will fail again however well the turbo was rebuilt, which is why renewing them on the car is part of the job.

Your unit is stripped to component level and measured before anything is ordered, so what gets replaced is decided by what the parts show rather than by a kit list. Bearings, seals and wear components are renewed to genuine OE or OEM-grade specification, uprated where the original design is the reason it failed. The wastegate mechanism is rebuilt, the rotating assembly balanced, and the unit run on the bench before it is boxed.

Which Bank Has Failed? What to Establish Before Anything Comes Off

A good part of this can be answered on the driveway.

• Follow the oil, not the noise. Localising a whine by ear across two turbos is unreliable; oil is not. Open the charge pipework on each side and compare — misting in one bank against a dry one on the other is the clearest indicator available without tools.
• A boost code names the system, not the part. An underboost code means the engine did not reach target, and a split hose or leaking joint anywhere in the pipework reads the same way at a fraction of the work. Pressure-test before committing to removal.
• Check play with the pipework off. Each compressor wheel should turn freely without touching its housing, with very little radial movement, and the wastegate rod should move without binding.

If that does not settle it, the fault detail is covered in more depth on our BorgWarner K03 and K04 turbo repair page.

This Is the S4 K03 Rebuild, Not the RS4

The B5 RS4 is the S4’s better-known relation and a different specification of car. Built for a short window at the turn of the millennium, it does not run K03 turbochargers at all — it runs K04-family units, and a listing claiming to cover both is describing neither accurately.

This rebuild is for the K03 turbochargers fitted to the 1997–2002 S4. References cross between models of that period, which is why the badge on the boot is the weakest evidence available. Read the reference stamped on your own casing, photograph it, and send it with your order.

How the Mail-In Rebuild Works

1. Remove the turbocharger from the car, or have your garage do it, and send it complete, with the wastegate actuator still attached where possible.
2. Order the rebuild here for the unit you are sending, or complete the online Repair Form first if you would rather we saw the details before you commit.
3. Pack it to survive the post. A rigid box, packed so nothing shifts, with the oil feed and return openings and both inlets covered. Put a note inside with your name, the vehicle and the symptom.
4. Post it to the workshop — Unit 102 Reaver House, 12 East Street, Epsom, Surrey, KT17 1HX, United Kingdom.
5. We strip, measure and report, then rebuild, balance and bench-test. Bench time is typically two to three working days from arrival; transit at both ends sits outside that.
6. Your own unit comes back, ready to refit. Change the engine oil and filter, prime the oil feed, and let the engine idle before it is driven.

Start Your S4 Turbocharger Rebuild

Add the rebuild to your basket for each unit you are sending — one per turbocharger, two if the pair is coming — or start with the online Repair Form and send us the reference from the casing. Every rebuild carries a lifetime, unlimited-mileage warranty, and the turbocharger that comes back is the one you sent. Rebuilds for other makes and models sit under turbocharger repairs.