Weird 580EX II snafu

I have two 580EX II flashes. About a month ago got them back from Canon repares. One is working just fine. The other stopped working in ETTL. No matter what I do in terms of the settings only pre-flash fires. It does work fine in manual mode. What could be the problem? Wonder if anyone else ever had this prob.
I'm guessing I'd have to send it back to Canon.
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skysi

There is a setting that turns the flash output off and only the preflash will fire to control other flash and set exposure. You can read about that in the manual. Find one online here if you don't have one http://eosdoc.com/manu​als/?q=580EXII* check page 39

I know about that setting. The problem is the flash is set to fire. BTW, both flashes are set identically.

I just looked again and found out that the flash actually drains the batteries as if it was a half power power shot. The red light takes a second to come on. The output however is no more than pre-flash.