If the EGR valve sticks in the open position, it will make the engine idle very roughly and/ or stall at idle.
If the EGR valve sticks in the closed position or fails to open, it will cause detonation (rattling noise on acceleration) and above normal cylinder temperatures which can lead to internal mechanical damage to pistons, valves, and cylinder heads. It will also result in a dramatic increase in oxides of nitrogen emissions, which will cause the vehicle to fail state emissions testing.
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