Check engine light can be on for many different reasons. If the check engine light went on after installing eFlexFuel, there is a reason to suspect that the device is not properly installed.
How to repair
First check the error code
If the problem is related to ignition, there is a reason to suspect that the wiring harness is not connected properly
Check error codes with the eFlexFuel App
Check that the ethanol content is correct
Check the temperatures to see that they are correct
Make sure that the ground wire is properly connected. Vehicle's battery (-) is the best place for a ground wire
Make sure that each connector on the cylinder and the terminals are properly connected
Make sure that the polarities of male connectors on the cylinders are correct (multimeter part in the instruction manual
Check that the connector pins are straight
The control unit has been corrupted and needs to be replaced