Symptoms
Rough or shaky cold idle, a misfire code, hesitation under light load, or gradually worse fuel economy without an obvious cause.
Likely cause
Port injection sprays fuel across the valve and keeps it clean; direct injection does not. Engines that combine both systems, or that use port injection alone, do not share the mechanism.
How should it be checked?
Confirm whether the engine is direct-injection only before assuming it applies. Cleaning is done by walnut blasting or chemical treatment, and the engine's design determines which is appropriate.
How can you check it yourself?
Start it from properly cold and watch the first thirty seconds. A rough idle that smooths out as it warms is the pattern to notice.
The question to ask the shop
Have the intake valves ever been cleaned, and does this engine use direct injection alone or direct plus port injection?