The chips are laid out in banks of three and there are two heartbeat signals one going beginning to end, the other end to beginning. Its actually pretty easy to chase down the problem with a decent multimeter and some patience. You can home in on the bank of chips that is having the problem and then do some resistance and other tests to further pinpoint the issue. The s9 is VERY similar in its layout and troubleshooting can be done in much the same way using the diagnostic points that are located all over the board. Just be VERY careful not to short a test point to a heatsink or you can kill the whole board rather easily.
Heh. I sense the voice of experience.
The other thing which is obvious, depending on your environment, is remove the boards and have a look. I have one board that is unusable - it looks like somehow moisture got in at the fan entry side (possibly a very quickly deceased large bug) and rotted half of the tracks around the "middle" of the chain. If I baby the voltages and speeds, I can get half that board working - but it certainly isn't a DIY effort.
These are tiny chips and pretty much every connection needs to work for the board as a whole to work. Send it to a repair shop and you'll have a far more consistent experience than probing with a multimeter (not that I'm saying that Fanatic isn't right - it absolutely is possible to diagnose if you have the time and no perceived value to the boards)