600ex ETTL not illuminating subject but TTL mode does illuminate

[hoppertunity]

TTL works perfect with Canon Mark iii...illuminates everything. Super bright

When I turn to ETTL nothing I do works. No exposure settings change the outcome. Images are super dark. 

Things I have done.

1. reset speedlight 

2. adjusted cameras exposure settings

3. made sure the flash firing is set to ENABLED 

Please help...

Also I know there is already a thread similar to this where ETTL was not illuminating the subject however nothing in there helped and does not address the issue where the speedlite works in TTL just fine. 

I am assuming the issue has something to do with the main flash not firing. 

The Speedlite is brand new out of the box. 

[MikeSowsun · Accepted solution]

Are you able to select both ETTL and TTL? You should not be able to select TTL mode. ETTL is for digital exposures and TTL (or ATTL) is for film exposure. (your flash has backwards compatibility for film cameras) TTL should be grayed out and not selectable. 

It sounds like you having a contact issues and the camera and flash are not communicating properly. When this happens, the flash may not sense the ETTL connection and is reverts to TTL mode because it thinks it is attached to a film camera. 

Try inspecting or cleaning both the camera and flash connections.

A momentary communication error could also cause a Custom Function setting problem. Try clearing the flash custom functions to see if ETTL functionality returns.