Q: Just drained/replaced coolant and now the temp gauge doesn't work

Some vehicles have air purge points that must be opened when coolant is re-filled. If you don’t open the purge points to force the air out as you add coolant in, air can get trapped in cooling passageways and that trapped air prevents normal coolant circulation. Related to this, it is possible that the tip of the temperature sensor could be sitting in a trapped air pocket, thus not contacting engine coolant at all, and therefore the sensor is simply not able to "read" the temperature of the coolant. If there is air trapped blocking coolant flow, and no reporting of engine temperature, you could in fact be overheating "parts" of the engine. If you suspect trapped air, it is actually possible to dislodge the air, and get it "moved" to the top tank of the radiator where it is not blocking flow, by simply swerving sharply, from side to side, as you drive. Doing this at low speed is fine as it is the suddenness and severity of the swerve that works, not the forward rate of travel. Mechanical force pushes coolant around and breaks the air bubble blockage and, again, the air gets to the top of the radiator. That’s typically how air gets out as drivers simply drive the car and the problem takes care of itself. Of course, coolant from the recovery tank is drawn into the radiator, due to the void in the top tank of the radiator and the system "rights" itself (be sure to top off recovery tank). If after a very brief interval (if this problem doesn’t resolve itself quickly, get assistance) you still see no temperature reading simply request a temperature switch/gauge diagnostic and the responding certified mechanic will resolve this.