A test method that will age coolants in a manner representative of coolant aging in the field is vital to the development of coolants that meet expected performance criteria. In an attempt to develop such a test, two approaches were taken. A phosphate buffered, molybdate/nitrite containing, propylene glycol based, heavy-duty coolant was aged in the laboratory using both a flow stand and a reflux apparatus. Samples of the coolant were taken at various time intervals during both tests. The samples were analyzed to determine glycol degradation product, sulfate and corrosion product accumulation, additive depletion, and pH changes. The results were compared to actual field results for the same coolant to determine which of the two approaches best simulated coolant degradation in the field. The flow stand appeared to best simulate actual field results.