coman.john@gmail.com

I have a 2005 A4 1.8T , stage 1 chipped by previous owner. Have been experiencing a P0299 "turbo underboost code" for a while. Initially came on at high rev acceleration on highway when gear shifted. The acceleration hesitated (like a hiccup) for a second, then light came on. I initially reset the CEL but it keeps coming back when accelerating at high revs going up hill.

Initially no loss of power, however recently noticed loss of boost (drives like it has no turbo) and also worse gas mileage.

Had the spark plugs initially switched, had shop check for boost leak (none apparently) and they also checked the turbo sensor (not sure exactly what that is).

Went to new shop (did not trust old one) and they diagnosed it as "most likely faulty n75 boost pressure valve". I am somewhat doubtful based on other people's reports on here and thought it would be more likely to be diverter valve.

Any thoughts on whether the boost pressure valve could be causing lower gas mileage /other symptoms?

lyleswk

Sounds similar to what I had with a 225hp TT which turned out to be a combination of diverter valve and a vacuum leak. controlling the DV. It is a cheap start to troubleshooting. The N75 is pretty expensive...

Nausforatu

I had the same code p0299 after I got stage 1 done. The top 3 causes according to my mechanic were either turbocharger/turbo inlet hose/N75 valve. I replaced the turbo hose and it didn't do anything then the N75 fixed my problem. Also this is only if you DON'T have a boost/vac leak. And they aren't that expensive. Under 100$ is pretty cheap for a sensor IMO. Genuine Audi part is 98.00 on ECS Tuning. On Sale right now for 49.95. Or the URO brand is 61.97. The replacement takes about 30 minutes if you know what you are doing, plan on like an hour or more if you don't. There is a write up for it on this sites tech forums. AudiWorld Tech Articles