2015 Audi Q5 - P0299 Underboost Condition - Turbocharger issues

Question

My Q5 is feeling very slow lately. When I'm merging onto the highway, the EPC light flashes and it feels like the car has no turbo at all. If I restart the car, the light goes away and the power comes back for a little bit, but then it fails again under load. Code is P0299.

I checked the air intake piping and everything seems tight. I don't hear any loud whistling or boost leaks. My mechanic thought it might be the diverter valve, but I wanted to get a second opinion before I start throwing parts at it.

MountainMechMo · Accepted Answer

✅ Confirmed fix: The Diverter Valve (N249) diaphragm had a small tear, allowing boost pressure to leak back to the intake side. Replaced the old diaphragm-style valve with the updated piston-style Pierburg valve (Revision D). This fixed the underboost issue and the car now holds steady boost pressure through the entire RPM range.

CoolantKai · Answer

The wastegate flapper on these IHI turbos is notorious for wearing out. If you reach down to the turbo (wait for it to cool!) and can jiggle the wastegate linkage and it feels loose or rattles, that's your P0299. It's not holding the gate closed, so boost escapes straight out the exhaust.

2015 Audi Q5 - P0299 Underboost Condition - Turbocharger issues

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