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Most likely it's a faulty thermostat, especially if your temperature doesn't warm up to operating temp right in the middle of the guage within 2 - 3 miles of beginning to drive and stay in the middle at all times for the rest of the drive.
Also check the temp sensor in the coolant flange on the side of the cylinder head. If it's black or green with a yellow dot replace it as they are a known issue and have an extremely high failure rate.
99% of the time it's the thermostat though.
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