An internal combustion engine having valve deactivation capability for at least one cylinder, or for an entire bank of cylinders in a multiple-bank engine, is provided with a temperature probe in the exhaust stream of that cylinder or cylinder bank. The temperature probe is connected to an Engine Control Module programmed to determine the rate of temperature change during valve deactivation, to compare the determined value to an acceptable range of rates, and to signal, when the rate exceeds an acceptable range. When the valves of a cylinder are properly deactivated, the temperature in the exhaust pipe decreases slowly. However, when valve deactivation fails, the cylinders pump non-combusted air from the intake manifold into the exhaust manifold, resulting in a relatively rapid decrease in exhaust temperature. This abnormal rate of temperature decrease thus can be used as an indicator of failure of the VVA system.