A diagnostic test is conducted by operating an engine-generator set at a plurality of reduced speeds. At each of those speeds, operating parameters of the engine are sensed and analyzed to detect an unsatisfactory operating condition. For example, such operating parameters may include speed of the engine-generator set, engine oil pressure and level, engine temperature, coolant level, and battery voltage. When a transition in engine speed occurs, a throttle test also may be conducted to measure how quickly and smoothly the engine reaches the new speed. After the engine reaches the normal operating speed, the generator is activated to produce electricity. One or more characteristic of that electricity, such as voltage, current and frequency, are sensed and evaluated to detect an unsatisfactory operating condition.