Winston 66, wonders.
As I understand it with my simple mind, I wonder if the perstistant (error code) ? problem is actually the knock sensors trying to tell you that there is a problem in the operation of the motor?
For instance , have you done any other mechanical checks on the motor Ie. compression, injector viabiliy, etc and or an actual check of the iginition timing as these sensors are supposed to detect pre detonation in the cylinders etc and is the fault code alerting you to an actual motor malfunction problem or is it telling you that the sensors and the associated wiring is not doing its job properly.
I wonder if there is or has been a noticable change in how the motor performes ie. starting, smooth running, power etc.
Have you checked when shortly after starting a cold motor that the individual exhaust pipes are at the same tempreature (I call this a finger test) but be carefull not to burn yourself.
This indicates that the individual cylinders are all firing and operating ok any cylinder that was down on temperature could indicate a potential problem with possibly iginition (spark plug), or fuel (Blocked or damaged injector), or compromised fuel supply (blocked filter).
This might possibly give you somewhere else to go, or to look at.
Hope that these suggestions are of some use.
Please keep us informed.
Cheers Winston 66 Northampton