Is it not working at all, or just not accurate?
Check for power to the gauge. Some operate at reduced voltage (around 5 volts). So it is possible the transformer could also be the problem.
Check your sender by measuring the resistance across the terminals (or terminal to body). Resistance should decrease as temperature increases. Different models vary, but normally when cold the sensor should be a few thousand ohms, and about 200 ohms when hot.
If possible - get the manufacturer (VDO?) details off your gauge & sender and look up the specs on them.
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