2015 GMC Terrain 'System Too Rich' Error Code After Rear-End Collision

My 2015 GMC Terrain was involved in a rear-end collision on September 24, 2018. The damage was approximately $2,500 and occurred near the rear exhaust area. The vehicle has 36,101 miles and had no prior issues before the accident. Since then, it has been experiencing intermittent stalling at traffic lights or during low-speed driving. The onboard diagnostic system is showing a 'system too rich' error code. A technician has suggested potential causes including the catalytic converter, fuel injectors, and several other components. The impact occurred in the rear section where both the exhaust system and a sensor are located on the right rear bumper. What does 'system too rich' mean in this context? Which engine parts or systems could be affected by this fault?