Powy

Hello all I have an 03 3 litre diesel 1KZTE or KZN16R ????, recently the engine light came on and the engine went into limp mode (turn off and on again it disappears), happened a few times and only seems to happen once the engine has warmed up. OBD scanner shows a P1123 error code - "valve for coolant circuit" Has anyone on here had similar issues and if so what is the common fault, what part needs replacing etc? TIA