Audi A4 B7: How to Replace Diverter Valve/Blow Off Valve
The P0299 code is indicated for turbo underboost, meaning your diverter or blow off valve needs to be replaced.
This article applies to the Audi A4 B7 (2005-2008).
If you have owned your A4 for any length of time, you might have experienced the P0299 CEL code, or you are getting it right now. Audi has since re-engineered and re-manufactured this little part called the diverter valve, or blow off valve. This valve is meant to release the pressure of the unused turbo power when you lift off the accelerator. If you are on a two lane road, pass a slow driver in front of you, and there is on coming traffic, you know exactly what this experience is. When your diverter valve isn't working, that pressure hangs out in your turbo charger and can cause damage. So while this CEL code and a bad diverter isn't necessarily a show stopper, it soon will be if you leave it unchecked. Your turbo charger will wear out soon and that is a big expense.
The nominal cost of this valve and the ease of install leaves no excuses to not get this taken care of sooner rather than later. You can easily do this job in under an hour in your own garage, but if you just don't want to mess with it, make sure that you take your A4 to a professional who has knowledge of VW and Audi, or you could end up paying twice as much in labor as your mechanic fumbles around trying to figure out how to fix it. Doing this job yourself is going to save you a bundle.
Materials Needed
- Torx bit driver (T5)
- Hydraulic floor jack
- 2 jack stands
- Chocks for rear wheels
Step 1 – Jack up your Audi and place on stands
Jack up the front of your A4 with a hydraulic floor jack in the approved locations. Place on jack stands in the recommended locations as well. For this job, near either of the front sway bar is a good location. Keep your hydraulic floor jack in contact with the approved jacking point for safety. Chock the rear wheels for safety.
Step 2 – Locate the diverter valve under the turbo
Your diverter valve is located on the exhaust manifold under the turbo. It is mounted with three T5 bolts; the top one is in a tight place, so you will want to use a small driver to get these out.
Step 3 – Remove the old diverter valve
There are three hex nuts that mount the diverter valve in place. There is also a wiring harness that needs to be disconnected first. Disconnect the wiring harness and then remove the three Torx nuts. You will need a smaller wrench to get the upper nut out. A ratcheting wrench works great for this. Once the three hex nuts are removed, the valve will simply pull right out. You can compare the old with the new to see the big difference in materials that Audi made to improve upon the design and durability of this part.
Figure 3. Diverter valve to be removed and replaced. 
Figure 4. If you inspect your valve, you may notice similar tears in this old design.
Step 4 – Install the new diverter valve
Installation of the new valve is just as simple as removing the old. This is truly a plug-and-play part. Insert the new valve into position and replace the three T5 bolts. They do not need to be torqued, but make sure they are nice and tight. Plug in the wiring harness and your job is done. Drop your car and take her for a test ride. Your CEL code should disappear and that familiar power should be back at your beck and call.
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