Since you say that it happened when you switched from PLA to ABS I would guess it's simply left over ABS in there that is blocking it for you. Try printing a small throw away print at ABS temperature to get rid of the remnants of ABS and it should work better for you.
There are 10 ways to fix this. Use google to search these groups and you will learn much. The above two methods combined may be enough.
Actually, I tried searching this forum and couldn’t find anything specific to clogging on an Ultimaker 2. There must be a procedure in changing from ABS to PLA without having to take apart the nossle afterwards.
Whouldn’t it be a good opportunity to sum up the 10 best ways to de-clog an Ultimaker 2?
Whouldn’t it be a good opportunity to sum up the 10 best ways to de-clog an Ultimaker 2?
True - most of the advice is for the UM1 but the advice is almost identical.
Someone already posted the 10 best ways. I've posted 10 good ways. I don't want to do it every day. Sorry.
Did you try bristle from a brush while hot?
Another nice way posted this week...
Remove the bowden tube from either end (not both ends) preferably the head end. Push the PLA in while at 180C. Then cool to 90C exactly. Wait about 2 minutes. Then pull hard on the PLA. The PLA will be *almost* solid and will pull out gunk and dirt and ABS with it.
Repeat this procedure 2 or 3 times or until the clog is gone. Then replace the bowden tube back on.
Remove the bowden tube from either end (not both ends) preferably the head end. Push the PLA in while at 180C. Then cool to 90C exactly. Wait about 2 minutes. Then pull hard on the PLA. The PLA will be *almost* solid and will pull out gunk and dirt and ABS with it.
Repeat this procedure 2 or 3 times or until the clog is gone. Then replace the bowden tube back on.
That actually sounds like a pretty good idea (in theory).
That actually sounds like a pretty good idea (in theory).
It is, indeed! 
last week I solved a serious clogging problem this way. There was a crust of carbonized material into the nozzle of my UM2. I had to repeat the procedure about 15 times, but now it's completely clean. The PLA flows like butter.
I had a clogged hotend last night.
Here is what I did because nothing else was working.
(1) Position the hot-end towards the front of the cabinet to make it easier to work on.
"gr5-simply turn off power and push the head to wherever you want it."
(2) Take the 2 fan screws out on both sides of the hot-end and swing the fans and bracket to the back, being careful that the wires don't touch the heating block.
(3) Unscrew all 4 long screws from the top that support the entire black box and slide out the hot-
end carefully while it is attached to the aluminum bracket.
(4) Take out the 2 screws that hold together the two aluminum brackets that hold tension between the spring that is around the nylon fitting connected to the hot-end. Once removed, take the spring and bracket off so you can only see the nylon going into the hotend.
(5) (Being very careful) Turn the printer back on and go into the settings to heat hot-end to 190C-200C (This was for PLA clog, not sure about ABS) and then remove the nylon fitting with needle nose pliers or the tool of choice. Then gently stick a straightened paperclip into the top of the hot-end where the nylon came out and twirl it around to get the clog out.
I pulled out a clog in the nylon and two clumps from the hot-end.
This may seem like a pain to do, but it is fairly easy and the only real way to know that you removed the clog.
Love it! One tiny change - for step one - simply turn off power and push the head to wherever you want it.
The nice thing about this is that you are less likely to damage the bowden tube this way.
It is definitely harder to do a clog removal on the UM2 than the UM1 and it's happening a lot more often for me. I haven't had a clog in 5 months with my UM1 after switching the fan to blow through the aluminum plate. I've already had 4 clogs in the few weeks I've had my UM2 and I've only used PLA the entire time. I'm wishing they had used an E3D nozzle, instead. This new nozzle design isn't working as well as the old one for me. Other than the frequent plugs, I'm loving the UM2.
I haven't had any plugging on my UM2 hot end at all, and I've been pushing it pretty hard, including doing prints with lots of retraction.
I have had it fail to feed fresh PLA into the head during loading a couple of times - I think the tip of the filament is sometimes catching on something at the end of the Bowden, so that the extruder just sit and clicks and never gets the plastic all the way into the head. When that happens, removing and reinserting the filament has worked to fix it every time.
I've also had zero clogs. Is your 3rd fan working? Or is it like illuminarti said and only happening when you change filament?
Actually what illuminarti describes can also happen after a print finishes because it does extra retraction then and then might not re-insert properly.
The plugs happen on prints longer than 16-18 hours for me. It seems the heat from the block is creeping too far up the feed path. The filament plugs up at the bottom of the nylon area. I'll have to check the third fan on my next print. I've also had the problem illuminarti described where it takes 2 or 3 attempts to get the filament to feed into the head because it's catching on something in the head.
Yes, does sound a bit like thermal creep. The third fan should be on all the time - as soon as the printer is powered on - before the lights even.
I've done 44+ hour prints at 230° with hundreds of thousands of retractions and not had any problems. YMMV, of course, but it provides a benchmark that shows that it *is* possible with good filament and everything set up right.
I think my clog was due to changing my material from one color to another using PLA for both, when I didn't allow enough plastic to come out of the nozzle to clear out any leftovers from the previous color. I got worried about the clicking sound when the extruder slips from too much pressure. Since then I am allowing plenty of material to ooze out when my material is being changed. (no more issues yet)
Not to hijack the thread but both of my side fans haven't worked since I first turned on my UM2. Maybe I need to check the board that they are plugged into? When I had the hot-end apart, I checked to make sure there weren't any frays or damage to the wires. Is there a setting in Cura or on the Controller that allows me to turn these on? My prints have been coming out fine without, but I think they could be much better.
If you have selected 'enable cooling fan' in the advanced settings tab, then the side fans should come on after the first few layers.
You can definitely change the fan speed from the Tune menu during a print - I don't remember if you can turn the fan on manually from the control panel outside of print, and I'm not at my printer at the moment.
If you checked the connectors in the mesh above the print head and they are ok, then the next thing to check would be where the fan lead connects to the electronics under the printer. The two side fans are wired in series, so they should either both come on, or neither. If it's not the wiring, then maybe one of the fans is bad... but there have been quite a few wiring issues, so that's probably it. I think that these fans will also not run if the polarity is reversed, so that's another possibility if everything checks out. My third fan had the polarity reversed.
Several people. SEVERAL (including me) have had the side fans simply not connected. The connectors pull out during shipping. YOu don't need any tools. Just slide up the net mesh above the print head. Slide it up the cable until you expose the wiring inside. There are 3 fan connectors - they all have red/black twisted wires going to the 3 fans.
The side fans connectors (coming from inside the UM) have a different color and the side fans have a common wire connecting them together. These connectors pop out during shipment easily.
Waiting for the 10th layer of a print before the fan is on is too time consuming. Instead print a random print and immediately go to tune menu (before it starts printing) and adjust fan there. You don't even need filament in the machine. You don't need to level. YOu don't need to start the print - just go to tune menu and it will not start printing until you leave the tune menu.
Thanks for the heads-up gr5!
I pulled back the mesh/netting and nothing appeared to be wrong, then I disconnected the white connectors and found that at least one pin was pulled out just enough to severe the connection to the fans.
I did need a tool(needle-nose pliers to pull pins back), but it was still simple.
I should have noticed the connectors located in the mesh without being told.
Both fans are working as they should now!
I must cleaning my nozzle too! How can I remove the bowden tube on the head end??
To remove the bowden you remove the red horse shoe looking thing on the coupler, then press the coupler down and hold it against the black housing while pulling the bowden out.
thx!! It worked with the 180°-90° method.
The 290° to 90° method worked fantastic for me! Thank you, I was pulling my hair out. And everyone is right the Ultimaker V1 was a lot easier to clear a clog and work with in general but the UM2 is much more automated and quiet which I quite like 
Just did the cleaning method 180 to 90 degrees, cleared my nozzle 
lets hope that this was the cause for my under extrusion and skipping problems! Going to test it out. Thanks for the info in this thread.
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braddock 1
I've been having a lot of clogging issues.
I've found the bristles off the brush I clean my BBQ with are a good diameter for cleaning the nozzle.
Heat the nozzle to 260 and poke one of these bristles up there... when it un blocks, it'll run down the wire to your finger.
Start a new print, and leave it set on 260, even if it doesn't extrude cleanly at first, let it run for a good ten minutes.
This has been working for me.
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