Some Focus ST vehicles, whether modified or stock, have experienced issues with rough idling, stalling, and overall bizarre engine behavior. This may or may not result in a diagnostic trouble code, making these intermittent issues very difficult to troubleshoot and diagnose.
The cause of these problems has been determined to be a manufacturing issue with the wiring harness splices.
Official recall notices were sent out towards the end of September 2014. You can also check your VIN against the recall to see if your vehicle is affected by visiting either the Ford recall search page: [LINK] or the NHTSA recall search page [LINK].