They are supposed to be the experts so if they can’t identify the root cause of the issue and then resolve the problem,
who how are owners supposed to cost effectively repair their car? Such is a sad indictment on the quality of the product Holden has designed.
In any case, you’d think the dealer should be able to sort out whether it’s a mechanical or electrical issue with the EPS, without huge drama. But since they have no clue, I’d look at it myself. I’d disconnect the EPS connector so (presumably) the system can‘t provide any steering assistance and reverts to an unassisted mechanical system. Then I’d go for a drive (knowing you won’t have any assistance) and assess the behaviour. If the car steers well and doesn’t vibrate or wander everywhere, you can be certain it ain’t a mechanical issue. Such would then mean it‘s a control systems issue with the EPS system itself. As such, you can then reconnect the plug, clear the fault codes and then do a recalibration of the various sensors in the hope that sorts out the system behaviour.
But really, I’d have thought there would be a way to passivate EPS system assistance via GDS, so one doesn’t have to disconnect the plugs, and then test the mechanical behaviour of the steering before doing any calibration. Maybe there is but if so why didn’t the Holden techs do just that?
Hopefully a calibration will sort this out…
Just keep in mind that EPS systems are rather sophisticated and complex. BMW for example uses them EPS to also provide vibrating haptic feedback when you wander out of your lane (analogous to stick shake on a plane)… So unless aerospace levels of system design and verification were used in their design, there can be ghosts in the machine which provide haptic feedback when it’s not required. Maybe that’s what you’re suffering