Audi Fault Codes, Part 0299

I had my first couple of codes thrown with a CEL back around 90,000mi. The fun thing with codes is they can have multiple causes and in fact the online shop manual I started using takes feedback from users as to what actually resolved a particular issue in order to help guide others. A pretty cool feature, although I still prefer a traditional shop manual I don't have to log into.

I did a pretty basic tune-up plus new coil packs and the car was running well. But then it threw a new code: 0299. This is for low boost from a turbo or supercharger. If the ECU calls for boost and the actual boost is > 2psi lower than expected you get this (per reading online). There are a myriad of possible causes, of course, and online searches for this error will turn up lots of folks having a tough time resolving it.

Handy part numbers:

Turbo Diverter Valve: 06H145710D. (OEM aftermarket runs around $64)

Boost sensor: Bosch 038906051E (OEM aftermarket runs around $23)

Turbo Boost Solenoid: Pierburg W0133-1916650

PCV Valve assembly w/hoses: Hengst 06H103495AKKT

I had purchased a new PCV/oil-air seperator previously, but not installed it yet. I was waiting for the codes that indicated it was an issue to return - and they hadn't.

I started reading up on the turbocharger system, having little experience with modern implementations. Common possible causes include the various sensors/solenoids (boost sensor, diverter valve, etc) and range in cost from $20 all the way to replacing the turbocharger.

I replaced the PCV as a start since I had it on the shelf, the car was now at about 94,500mi and it is an important part of the system, having to manage both crankcase pressure as well as waste boost. 20/20 hindsight being what it is, it was the right thing to replace except I did not replace the hoses to and from the unit. For this rookie error in judgement, I would have to suffer a bit. There are kits with the PCV assembly and both hoses. For a reason.

I replaced the bypass valve next, it's located on the front of the engine and is accessible from beneath and seems to be a common failure part. The code returned.

I replaced the boost sensor; it's on the front left, mounted to the oulet hose from the intercooler. Maybe it was giving the ECU bad readings? Nope. The code returned.

I replaced the turbo boost diverter valve (the thing with 3 little vacuum lines coming off of it). It is located next to the bypass valve. The outside was coated in oil and dirt, perhaps one of the hoses was leaking or the device itself was leaking? Hard for me to say. I put the new one in, but the 0299 code returned.

I had been hearing a small exhaust leak for a while, at least a year, and decided to deal with it in hopes that would help somehow. At least the car would make the proper noises at the proper ends while I got the issue sorted out.

The clue to my mistake, I think, was when I took the car out for a test drive after replacing the turbo boost solenoid. The hose running across the top of the engine from the PCV valve to the intake manifold popped off at the intake manifold. I had never touched it on that end. With a big enough leak the car will barely idle, and perhaps a smaller leak under boost could cause a low boost situation. A bit of a guess from me, but I think that is what was happening.

It's difficult to test some of these systems without a fancier $$ code reader and/or a high pressure smoke machine. My little $50 smoke machine is fine for evap hoses, intakes, exhausts and things like that. But simulating high boost scenarios requires quite a bit more pressure.

Net net, I just took an 1,100mi road trip and the CEL has not returned. The car races to redline like a proper sports car again now and feels much, much, better.

Pics and links:

This is the boost sensor, $25 and probably not a bad idea to replace after 12 years.

Like most things these days, you need to be under the car to access these sensors. You can see the oily film that had attracted dirt here. Certainly looks like mist blowing out of something as opposed to someone spilling oil.

Only real tips for replacing the diverter and boost solenoid are to remove the top splash shield for better acess up top, and then of course removing the lower splash shield beneath the car. The boost solenoid is a bit of a pain because the metal inserts that take the bolt pressure just slide in and any sort of movement as you re-thread the bolt will make them fall out. Good times.

Removing the PCV valve is just taking out the coil packs, then a pair of hoses. Squeeze the top one to disconnect it, and the other hose from the turbo housing sits in the PCV housing. They use is T25 bolts, which conveniently match the splash shield. Obviously don’t get any junk in the engine as you do this or you’re going to have a bad time(tm).

All buttoned up, easy job but I still managed fail since I did not replace that top hose. Live and learn.