Orriss,
Your Engine Control Module (ECM) has detected that the throttle control motor relay is stuck ON.
Power supply for the throttle control motor is provided to the ECM via the throttle control motor relay.
The throttle control motor relay ON/OFF is controlled by the ECM.
When the ignition switch is turned ON, the ECM sends an ON signal to throttle control motor relay and battery voltage is provided to the ECM.
When the ignition switch is turned OFF, the ECM sends an OFF signal to throttle control motor relay and battery voltage is not provided to the ECM.
This will explain why you are going onto limp mode.
Just because the relay is stuck on does not necessarily mean that the relay is faulty.
Possible causes including the relay, could be dirty connections, faulty wiring, or even ECU or IPDM.
Alex.