Oh, sorry! I posted before reading your latest update!

 

When you hit the start button on your bike, a computer message is sent from the right hand switch gear to a module called a BCM(Body Control module). The BCM is the major controller of most electrical items on your bike and in this instance the module is supposed to send voltage to your starter solenoid when you hit the start button which allows it to engage the starter and turn your bike over if relevant conditions are met, for example, you're not in gear with the side stand is down.
The amperage that the starter solenoid draws is monitored by the BCM and if it falls outside a certain range a code is set.
B2121 if it sees too little amperage being drawn & B2124 if it sees too much amperage being drawn.
It is a pretty simple check to verify which component is at fault and if that is the only code, then I'd suspect that the BCM isn't sending any voltage to the starter solenoid when you hit the start button and when it checks to see how much current is being drawn, it obviously sees too little being drawn and throws the code.
If it was a right switch gear issue or a canbus communication problem between the switch gear and the BCM, then the BCM wouldn't receive the message in the first place to make the decision to monitor the starter solenoid current draw.
The first check the shop should be doing is checking for voltage at the wire that goes to the starter solenoid from the BCM when the starter is pressed.
If no voltage is present then the next check would be to see if the BCM is sending voltage along that wire by measuring voltage at the relevant terminal coming out of the BCM. If voltage is coming out then there is likely a break in the wire to the start solenoid....never seen that so far.
If no voltage is measured coming out of the BCM when the starter is hit then BCM replacement is required. Seen that a few times although not a common issue.
I've seen a few switch gear issues but like I say, that may well not be the cause of your problem in this case.
It could be an intermittent issue, hence the shop not figuring it out but I'm surprised that the code hasn't pointed them in the right direction.

 

That's the most thorough explanation I've had yet! Thank you!

 

You're welcome and hope they get things sorted for you soon.

 

Great write-up!
Intermittent issues are a PITA to diagnose. That's one of the reasons every manufacturer of equipment with electronics has built in diagnostics that monitors performance and stores the results...such as HD diagnostic codes. There should be a fault procedure for every diagnostic code, or at least a prioritized list of likely contenders. BTW...a weak battery is extremely unlikely to cause an intermittent problem.

 

I didn't realize the starter was triggered through the BCM, but I don't have my service manual in front of me to look at. Good info.

 

I just got a notice in the mail today from Harley Davidson advising me there is a recall on the Low Rider ignition switch. It is a known issue and they will replace the switch and the steering lock and give you new keys.

If you haven;t received a notice yet give it a week or so and then call your dealer. I'll be calling them tomorrow to schedule a time to go in and have it replaced.

 

That's the first thing they did. Wasn't the problem.

 

I believe that's only on the newer bikes with the CAN-BUS (or whatever it's called) system. Your '09 wouldn't work that way.

 

'09? He said the bike was new and under 2500 miles....