The most frequent cause of the P0021 code is insufficient or contaminated engine oil affecting the hydraulic AVCS operation. To confirm this, check the engine oil dipstick for the correct fill level and inspect the oil's viscosity and color for signs of sludge or heavy contamination. If the oil is below the minimum mark or appears dark and gritty, it indicates that the hydraulic pressure required to actuate the oil control valve is compromised, leading to the timing error.
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