I replaced my temperature gauge sensor with a new one and now the gauge's needle peaks at just below the red zone when the engine warms up. I forget which vendor I ordered it from but it looked exactly like the old one except that the probe that goes inside the engine block is maybe a millimeter or two longer than the old one.

I thought I replenished any coolant lost during the change. I didn't drain the entire block; just above where that sensor sits. I put in the new coolant through the overflow tank.

The old sensor worked fine, BTW. I guess the easiest fix is to put the old sensor back in and see what happens. Is there something else I should be looking for?
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Mike F - 1984 244 DL - 317,000 miles
Original engine, transmission, drive train, starter
Undergoing reconstructive surgery with POR-15

1972 142 S - The Yellow Brick - 135,000 miles
All stock except for Weber Carb. B20B engine. M40