Q: Check engine light flashing at hwy speed

Have a certified technician start off by scanning the system for fault codes. When the check engine light is on faults will be stored in the system. These faults will give the technician a direction to take do diagnose the fault. Something like a misfire can have the check engine light on in a solid state when at idle, but flash at speed because the fault is seen more often. This can also lend to a hard start if all cylinders are not firing properly.Misfiring can be caused by ignition components such as bad spark plugs or a bad ignition coil. Misfires can also be caused by improperly firing fuel injectors, either because they are clogged or failed.

Have a certified technician check for misfires and isolate why the check engine light is on and flashing.