I bought a kit of 3000MHz Corsair DDR4, both sticks are detected and work fine with the default frequency/voltage and have done so for quite a while.  But if I try to load the XMP profile or use any clock above 2133MHz (after setting the voltage to 1.35 and restarting), the computer doesn't post and I have to reset CMOS to get back in.
 
Bios versions is 1.09.
 
So which part of my PC is broke and causing this? Is the RAM bad? Is the motherboard bad? Is the PSU bad?
 
The only thing I know is that at the default 2133MHz and auto voltage it all works fine, I have done stress tests, played dozens of hours of games, never any problems with all defaults...
 
Both the motherboard and the RAM are "new" (couple months old at this point) and bought at the same time.  I had this problem since I got the parts, but I need a PC and had no spare ITX motherboard or DDR4 RAM to sit around for weeks waiting for an RMA back then so just said fudge it and ran with the defaults.
 
Not that this is a particularly pressing issue, I bought the RAM on sale and it was cheaper than slower RAM, so I didn't really waste any money for faster clocks, but I would still like to use those clocks if I can.
 
Thanks

Sajin
What cpu are you using?

6600k, no OC.

Sajin
When setting the ram manually did you also set the correct timings?

I didn't touch the timings.
 
And by manually I didn't even go high, I would just go up one tick from 2133 to like 2400.

loveha
Just a weak IMC that can not go higher than its rated 2133MHz.

This is most likely the issue if the OP's situation doesn't improve after setting correct timings & adjusting VSA & VCCIO voltages. 

VCCIO & VSA voltages shouldn't exceed 1.35v.

Try setting all timings manually, or just first and second if you don't understand third timings. I have found some third timings are never correctly assessed by the BIOS. I can leave just two of them blank and it won't boot my 3600mhz with certain other timings set, leaving all thirds blank and it boots.

Another major factor can be that smaller refresh setting, not the thousands one but the hundreds one (I want to say tRFC). I can pull 270, the default is 374, but my XMP sets it at 600-something. Lowering this decreases interrupt delays and thus latency, but not all memory modules can handle it. There is a reason for each of these numbers, something about the number of consecutive bank or rank pulls and the time until they are available again. I read an extensive amount on DDR4 reasearch, but am a bit rusty on details a year later. I have the PDFs somewhere if you would like.