Then RMA the WHOLE memory kit. Do not allow them to replace only ONE stick, because memory needs to come together, matched, for compatibility, even when it is the same part number.
Long version of why you don't want them to replace just the faulty stick. Don't let them even try. INSIST on the whole kit being replaced even though it will mean not being able to use the system while you are waiting.
The odd man out, (Or, mixed memory)
While memory modules that did not come together in a matched set that was tested by the manufacturer to be compatible, certainly CAN still work together, often it does not. Right up front I'll tell you that if you are trying to get sticks to work in the same machine together that were purchased separately, even if they are otherwise identical according to the kit or model number or if they would seem to have identical timings and voltage requirements, there is a very good chance that you simply will not be able to do that. There is also a pretty fair chance that you might be able to if you are willing to take your time, listen to and understand what you are being told and follow the steps necessary to determining if they will "play nice" or not.
The exception in most cases will be that if the memory from both sets are the same speed and timings and both kits are within the JEDEC specifications for the default speed on that platform, so for example, 2666mhz on the latest Intel Z390 platform, 2133mhz on Ryzen first and second Gen platforms, then they stand a much better chance of working together but if they are higher speed kits the chances begin to diminish from what they might be at the low speed and loose timings end of the scale.
A word of advice. If you just purchased this memory, and for whatever reason you bought two separate sticks of the same memory instead of buying them together in a matched set, see if you can return them for a refund or credit towards buying a similar or same set of matched sticks that come together in a kit. It is ALWAYS better to have matched modules because from brand to brand, or even within the same brand, in fact, even when the part numbers are IDENTICAL, there can be anything from simply slightly different memory chips that were sourced from different bins at the end or beginning of a production run to entirely different configurations altogether even though the model numbers seem to be the same. Some manufacturers even reuse model numbers when they discontinue a product. Point being, memory is only the same for sure when all sticks came out of the same blister pack or packaging and were sold as a tested kit.
In order to determine if differences in the memory, or a need for increased voltage when using more than one stick (Especially if you are running three or more sticks) are responsible for the problems you are having you will always want to begin your troubleshooting process by attempting to boot the machine with only a single stick of memory installed. Also, for practically every consumer motherboard that's been sold since at least as far back as about 2014, the A2 memory slot which is the second slot over from the CPU socket, is THE slot that is most commonly designated for the installation of a single memory module. Slots A2 and B2 are almost always the slots specified in the motherboard memory population rules for use with two modules. If you need to install a third module I have no opinion on which of the remaining slots to use for that, but typically since the A1 slot is right next to the CPU socket and often interferes with the CPU cooler or fan, I'd say the B1 slot was probably just as good.
Honestly, I don't ever recommend that you HAVE three modules installed anyhow. Using memory in pairs is almost always a better option, except on boards that support triple channel memory population, so that normal dual channel operation will occur. And that's another thing. When it comes to memory there are no "single channel" or "dual channel" memory modules. There are ONLY memory modules and the motherboard and CPU architecture will determine whether or not dual, triple or quad channel operation is possible based on the architecture and how many modules are in use. Occasionally though there are situations where it might make sense to run three modules and some boards CAN use three modules in a FLEX type mode where two of the modules will operate in dual channel while the third oddball module will run in single channel. I'd avoid oddball configurations though if possible because many motherboards will simply run ALL modules in single channel mode when an odd number of modules are installed.
If you think you will ever need 16GB of memory, then buy 16GB of memory from the start so you can get it all in a matched set that has been tested,
and eliminate a lot of problems right from the start.
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