From the owner's manual: Limp Home is mentioned twice
"The Malfunction Indicator Light (MIL) for the engine management system illuminates when the ignition is switched ON (to indicate that it is working) but should not become illuminated when the engine is running. If the engine is running and there is a fault with the engine management system the MIL will be illuminated and the general warning symbol will flash. In such circumstances, the engine management system may switch to 'limp-home' mode so that the journeyman be completed, if the fault is not so severe that the engine will not run. If the MIL flashes when the ignition is switched ON contact an authorized Triumph dealer as soon as possible to have the situation rectified. In these circumstances the engine will not start."
"If there is a malfunction with the throttle control Indicator Light the (MIL) Malfunction becomes illuminated and one of the following engine conditions may occur:
▼ MIL illuminated, restricted engine RPM and throttle movement
▼ MIL illuminated, limp-home mode with the engine at a fast idle condition only"
On another note, if the codes don't clear with TuneECU it's because you have to perform a procedure like a switch check for the cruise control. That would be a P1575 fault.