"Check Engine" light on after flushing radiator

If you didn't bleed air as you refilled the coolant, you may have an airlock. On my Cummins ISB there are two small lines (hoses) coming from the block near the thermostat housing, they go above and behind the radiator, with a petcock on each. These vent the air as coolant is added. Open those until a small amount of coolant comes out...
Air trapped in the system can cause overheating.
Assuming (hershey is probably right) you did run the heater after refilling the coolant...