Because the gauge "failed" a test. Seemingly this points to a bad gauge. So you have to get to the gauge.
1. Disconnect battery and pull cluster (disconnect speedo and connectors.)
2. Take to bench, take out gauge and test or leave it in if possible.
3. Check if gauge is wired properly. Don't reinstall cluster until all is well with temp gauge. Also it's possible you will want to test other cluster functions...... other gauges, turn sig lamp, high beam lamp, etc. You can run all the gauges and lights with cluster plugged in, not installed (or installed with two screws), speedo not attached.
