I recently repaired a couple of key transmitters for Martin (Fintro11), so thought I'd pass the info along.
Symptoms:
Both transmitters LEDs would flash when buttons were pressed, but either no response from car or intermittent function.
Battery would quickly drain.
Failure:
Flexing of the circuit board from years of button presses caused broken solder joints on the rear of the board.
Repair:
First test all buttons with a DVM set to continuity and confirm function.
De-solder and remove battery holder from the rear of the board.
Inspect all solder joints for breaks (will require 30x magnification inspection scope or equivalent - cannot be seen with the naked eye).
Reflow all solder joints with flux, a fine tip iron and scope. A very steady hand and some basic knowledge of electronics soldering is required. Too much heat or flux will destroy components and/or circuit traces and stray solder will cause shorts.
After soldering is finished, gently clean the boards with isopropyl alcohol and a soft bristle brush to remove any debris and excess flux, then examine all joints under magnification to ensure proper repair.
Reattach battery holder. Enjoy your working key transmitter.
This picture shows some broken solder joints under 30x magnification.
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particlewave 
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