Symptoms

Although there is no single definitive indicator of a failed actuator there are a number of symptoms that are common to actuator failures:

Limp Mode AND DTC Codes P0299 and/or P0234 - Loss of power (limp mode) accompanied by the CEL/MIL and an Over Boost or Under Boost Condition or both. Circumstances:

• Sporadic or Intermittent of the above symptoms, usually cleared by shutting the engine off and restarting. The issue will re-occur with increased frequency (at first once every month and eventually daily).
• The above symptom occurs once engine warms up and clears after parked for a few hours and engine cools down.
• The above symptom occurs when engine is cold and can be cleared be restarting the engine after the engine warms up.

Diagnosis

If the symptoms are intermittent, there is no definitive diagnosis that can be performed on the vehicle; the actuator will need to be tested on the bench. Having said that there are a couple of easy tests that can be done on the spot which will pinpoint a failed actuator:

With the engine dust covered removed (2x10mm bolts) the turbo actuator is exposed and visible. Ensure engine is cold to avoid burns as the turbo gets very hot. Turn the ignition to ON but do not start the engine. Use your index finger to very gently (I can't stress the word GENTLY enough) either push or pull on the actuator arm. The actuator should react to this disturbance by opposing your movement which you will be able to feel or at least hear a buzzing or electronic mechanism sound.
If no reaction is felt or heard, the actuator definitely is NOT WORKING. If there is a movement or sound, the test is NOT CONCLUSIVE one way or the other. (Hence the intermittent issue)
If the arm freely moves up or down, THE GEARBOX IS DAMAGED, there should be resistance and the arm should NOT move up or down freely.

Not the Turbo?

In general Garrett turbine fitted to these engines is very robust. Even so, there is always a chance for failure regardless of the circumstances leading up to it. Here’s how to perform a quick check on the turbine itself:

Visually inspect (Inlet)

You should see absolutely no foreign materials in or around the turbine blades. That means, no pieces of the rubber seal, no pieces of plastic or paper or cloth etc.
There should be absolutely no aluminum shavings or any sign that the steal impeller has ever touched the aluminum housing around it. If any aluminum shavings or scratches are found, the turbine is damaged and needs to be replaced.
Ideally the turbine impellers should be clean and try, but this is never the case. You will observe some modest amount of oil present and the blades will be oily and somewhat dirty. Although that is a vague description, what you should NOT see, is caked on carbon build up (looks like black velvet layer), excessive amounts of oil, gummed up black oily material (oil + carbon mixture).

Mechanically inspect (Inlet)

With your fingers, very gently (stressing the word GENTLY), try to spin the impeller and check for shaft play; it should scroll smoothly and easily and there should be little to no shaft play, both in and out and side to side. If you have excessive play, you probably also have aluminum shavings and scarring on the housing, indicating the turbine is damaged.

Checking the Turbine Vanes.

Using a small flat head screwdriver remove the C-clip securing the turbo vanes leaver to the turbo actuator arm by prying it off. DO NOT USE PLIERS, using pliers will damage the actuator when pulling on the C-clip. DO NOT LOSE THE C-CLIP, DO NOT DROP IT. Once the actuator is de-coupled from the turbo, you may check the turbo vanes by moving the turbo vanes leaver by hand. The leaver should move freely with little to no resistance and should not feel scratchy, or notchy or sticky in any way.
A sticky leaver indicates the vanes are bogged down with carbon/rust/guck deposit. Turbo must be removed for cleaning or rebuilding. (not a service I offer)