Reduce the load before chasing a rating
A higher dollar bill does not automatically mean the HVAC system suddenly became inefficient. The bill is the product of energy consumption and the utility's rates and charges.
Start by comparing actual kWh or fuel usage with prior periods, then consider weather and household changes. Only after separating those factors can equipment operation be evaluated meaningfully.
A sudden unexplained increase in runtime, auxiliary heat, cycling or loss of comfort can point toward an HVAC issue that deserves diagnosis.