There is a forbidden air around Shimano STIs.
When asked for advice on reassembling an Ultegra STI lever,
a Shimano spokesman said
No one is going to be able to repair that shifter.
He has disassembled a unit that is fabricated in Osaka using
the very best of our SARA robots. (Shimano Automated Robotic Assembly).
In fact STIs come apart and go back together quite easily; if you can replace a head gasket on a Norton Commando, you'll be able to rebuild an STI. The real problem lies in the non-availability of spares. Which is really, really miserable of Shimano, especially when you see the spares list for something relatively mundane like a pedal. Hey, ho. If an internal component is broken or badly worn the only option is to canabalise another lever for parts. Don't throw that low mileage crash damaged lever away, get it touch and we'll buy it.
If your STI shifter is out of warranty then you've nothing to lose.
If you don't relish the task yourself,
get in touch.
If Shimano refuse your warranty claim, we can provide a report on the condition of the lever that may assist your case. Although, in fairness, it has to be said that Shimano usually accept faulty STIs without argument; maybe they know there's a problem.