Audi A4 B7 (2004–2008) K03 Turbocharger

What Comes Off the Car on a B7

On the 2.0 TFSI B7 the K03 is not a standalone turbocharger. It is removed — and sold, when people buy a new one — as a complete assembly, with the exhaust manifold, the pneumatic wastegate actuator and the diverter valve all part of the same item rather than separate parts bolted on afterwards.

Two things follow from that, and both matter before you order anything. The first is that removal is a manifold job as much as a turbo job: studs that have spent years heat-cycling, a heavy assembly to support as it comes away, and gasket faces that have to be clean when it goes back. Allow for the time, and have the gaskets and fasteners in hand before you start.

The second is that a fault sitting on the turbocharger may not be a fault in it. The diverter valve and the N75 boost-control valve travel with this assembly, but they are control components rather than part of the rotating assembly. They fail in their own right, they produce boost complaints that read exactly like a tired turbocharger, and unlike the turbocharger they can be tested on the car.

Worth Testing Before the Assembly Comes Off

An underboost or overboost code on its own is not evidence that the turbocharger is finished. On this engine the cheap checks come first, and there are three of them.

The diverter valve is a wear item across the VAG petrol range. One that has split or is leaking bleeds charge pressure straight back to the inlet, and the result is the flat, laggy performance owners reasonably describe as turbo failure. The N75 valve governs how much pressure reaches the wastegate actuator, so a lazy or partly blocked one gives boost that will not build, or will not hold once it does. Charge pipework and hose joints leak under pressure and read the same way again. All three can be settled without a spanner going near an exhaust stud.

What the cheap checks cannot explain is noise or smoke. A whine that rises and falls with boost, a rattle from the wastegate at light throttle, or blue smoke with oil in the charge pipes are wear inside the unit, and no valve test will move them. That is the point at which the assembly comes off and the turbocharger goes on the bench. Wider fault detail across this turbo family sits on our BorgWarner K03 and K04 turbo repair page.

What the Rebuild Puts Right

Your unit is stripped to component level and measured before any parts are ordered, because what a K03 actually needs is decided by wear figures rather than by the symptom that sent it in.

This application runs journal bearings, cooled by water as well as oil; those bearings and the thrust components are the usual source of shaft play and the whine with it. They are renewed to genuine OE or OEM-grade specification, along with the oil seals that begin passing oil into the exhaust or the inlet once bearing clearance has opened up. The wastegate mechanism is freed, its worn hardware replaced and its travel set, so the flap seats properly instead of shaking about in a slack pivot. The rotating assembly is then balanced, and the finished turbocharger is run up and tested before it is packed.

Turbine-housing cracking does turn up on this unit, and it is not something a bearing and seal rebuild can put right. If we find it on strip you are told, rather than having the unit quietly rebuilt around it. Every rebuild carries a lifetime, unlimited-mileage warranty, and this is not an exchange scheme — the turbocharger booked in under your name is the one that comes back to you.

If Your A4 Is a 2007 or 2008 Car

Those two years appear on two of our listings, because B7 production overlapped the arrival of its replacement. A 2008 registration on its own therefore does not settle which of the two is sitting on your driveway.

The identification plate on the turbocharger does. Photograph it, send the reference with your order, and we will confirm the listing against it before any work starts. If your car turns out to be the later generation, the A4 B8 K03 rebuild is the listing you want instead.

The same caution applies at the earlier end of the range. Our B6 listing runs to 2006 and this one starts in 2004, so a car registered in those years could be either shell — if yours is the earlier one, the B6 K03 rebuild is the listing that covers it. The plate on the unit settles it; the registration year does not.

Posting Your Unit In

1. Photograph the identification plate on the turbocharger before you pack it, and the assembly as a whole.
2. Send it complete — wastegate actuator and linkage attached, every port covered so nothing can get inside, and the assembly packed so it cannot shift in transit.
3. Address it to Unit 102 Reaver House, 12 East Street, Epsom, Surrey, KT17 1HX, United Kingdom, with a note giving your name, the car and what it was doing when it came off.
4. Refit with fresh oil and a new filter. Renew the oil feed and return lines while the assembly is off, and prime the feed before starting. A restricted feed will finish a newly rebuilt bearing as surely as it finished the last one.

If you would rather have the unit assessed before committing to the order, send your photographs and the symptom through the online Repair Form.

Order Your A4 B7 K03 Rebuild

Add the rebuild to your basket to book your turbocharger in, or start with the online Repair Form if you would like the identification confirmed first. The work is done in our own UK workshop by turbo specialists, and the unit is bench-tested before it is returned, ready to refit.

If the unit on your car turns out not to be this one, the rest of the turbochargers we rebuild are listed in the shop.