Hello and welcome to JA, yes the camshaft does need to come off to replace the VCT Phaser...usually this goes bad from either lack of maintenence...long intervals between oil changes..or using aftermarket oil filters, which break apart internally and clogg the oil passage to the VCT Phaser...replacing it is the only true way to fix it, once it has been starved of oil, it is damaged beyond saving.
I am a bit confused. Is the VCT phaser the solenoid or is it the the exhaust cam sprocket that fills with oil? Is the spring in the solenoid or the sprocket? When this systme goies bad, what parts need to be replaced (for ex. the valve cover gasket appears fine, but are there other seals, etc. that need to be replaced?)
they are one in the same, called VCT solenoid/phaser, the solenoid can be replaced seperately, but when the screen in the phaser is restricted with debris, the solenoid is not lubricated, causing it to stick. Both need to be replaced at the same time for a proper repair.
the valve cover gasket should be replaced anytime the cover is removed.
I still don't understand completely. When you said, 'both need to be replaced' do you mean 1) the solenoid and
2) the sprocket outside of the valve cover that the timing belt turns?
yes, that is correct, the VCT solenoid gets a signal from the PCM to allow or decrease the amount of oil that enters the phaser, this in turn advances or retards the timing.