What should the treatment be if a tooth with an old filling has mild pain on percussion and sensitivity to cold?

The answer varies from case to case, but if the cold sensitivity is brief and the tooth is vital, pulp capping or a replacement filling can be tried without root canal treatment. Radiological examination is also important. A periapical radiograph and, if considered necessary, CBCT can help with diagnosis. If the filling was placed recently, it is normal for cold sensitivity to continue for a while. If a long time has passed and it still persists, there may be a shortcoming in the adhesive protocol for the filling, or the tooth may have cervical sensitivity unrelated to the filling.
Sensitivity to percussion may indicate a need for root canal treatment in some cases, but not every instance of percussion sensitivity means that root canal treatment is necessary. Factors such as food impaction, sinusitis and referred pain can also cause sensitivity to percussion.