Common signs of refrigerant contamination in an HVAC system include decreased system performance, increased energy consumption, and unusual noises or vibrations. Other signs may include frozen coils, corrosion, or rust on system components, and refrigerant leaks. In some cases, contamination may also cause the system to malfunction or shut down entirely. If any of these signs are observed, it is essential to investigate and address the issue promptly to prevent further damage and system failure.