I had code P0021 also on my O6 3.5. Engine was missing very badly. Would have a hard time starting up after cranking for a long time. It felt like it had no power, as if it was only firing on 3 cylinders. After much troubleshooting, found out the Intake valve timing solenoid valve was gummed up with very gunky motor oil. Took it out and cleaned it with WD-40, tested the solenoid to make sure it operated OK and re-installed it. Immediately ran like a new one again. I changed the oil more often from then on.
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