I know this is an old thread but i have had this issue for a long time now (over a year). on My Focus TDCI 1.8 02 plate

always the same error when going into limp mode, P2339 cylinder 4 above knock threshold.

the problem i see is that i swapped injectors from cylinder 4 to cylinder 2 and the problem is still on cylinder 4 (which in my book rules out the injector) should the fault not follow the injector to number 2 not stay at number 4??

any new advice appreciated.

eX