I see your dilemma. The suffix letters refer to the version of the module, and apply to different A4 models that they are used on. You are assured of the AJ fitting and being the easiest to configure as it's already compatible with your remotes etc. Depending on what A4 models other suffix versions fit, later versions will work too, up to the B7 versions, and often have extended configurations available via coding and adaptations. The latest versions like the B series, may need different key remotes, again depending on if they are B6 or B7 modules. All B6 version modules will work without new key remotes. So, any module originally fitted to a B6 up to M.Y. 2005.5 will work without additional changes. Check with the dealer for what suffixes are original up to 2005.5. Regardless of the version used, the remotes will need to be adapted to the replacement module, using VCDS/VAG-COM or the dealer. The immobilizer function is not a CCM function so that won't be effected as long as the same remotes are used.
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