An internal combustion engine is selectively operative in a spark ignition combustion mode and a controlled auto-ignition combustion mode. An EGR valve operative to control flow of exhaust gas to an intake manifold is monitored, including commanding the EGR valve to a closed position and monitoring operation of the internal combustion engine. An intake manifold pressure due to a fresh air charge is estimated based upon the operation of the engine. The intake manifold pressure is measured, and the estimated intake manifold pressure is compared to the measured intake manifold pressure.