Seen this problem many times before. Odds are your magnasteer actuator is stuck. They either go and your assist is completely gone or in the full position. Since you can't feel the difference odds are that's where its at.

Perhaps it's working intermittently that's why it takes a few miles for the service light to illuminate or it just takes that long to realize the problem.

There is always the chance your ebcm is going south as well.

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