One problem related to check engine light on has been reported for the 2010 Mazda CX9. The most recently reported issues are listed below.
After recently receiving our vehicle back from a Mazda dealer for warranty repairs on the ptu and control arms, we were driving down the highway at 65 mph - initially going up a hill in the mountains. The car would not shift gears (an automatic) and then would not accelerate past 30 mph. No dummy lights lit whatsoever. We got to the top of the hill and having no place to pull off on the highway mountain shoulder, stayed in the lane of traffic with our hazards on. The down side of the hill was a 6% grade with a cliff on the other side of the guard rails. All of a sudden - the power steering went out and the car was hard to steer. There was still no place to pull over on the mountain highway, so we coasted. We then noticed the rpm's were at 0 and all the dummy lights that are on when the car ignition is in accessory were lit up. The car had turned off - while we were going 65 mph on a 6% grade. No warning - no sounds, smoke, dummy lights - just off. Very amazingly dangerous. We were very lucky to not die a fiery death. My husband was able to keep calm and thankfully the brakes worked - we were able to stop towards the bottom of the hill. We had the car towed as it wouldn't start/stay running. The diagnosis by the dealer (not the dealer who did the prior warranty work) was a failed engine rod. Would have been nice to get a check engine light before having the car completely shut off. We drove the vehicle for less than 100 miles after receiving back from the warranty work and now this.