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So Ive had my Rostock Max v2 for about a year now, ~18days of print time. Give or take a few weeks, I think. Its worked great up until recently.

Ive been having trouble maintaining hotend temperature sometimes, but not always. Its always a lower temperature, not an issue with fluctuation. Most times Ill just turn the bed off (Printing PLA, so after the first few layers, ive yet to notice a difference), and that will help, but not always. I really first noticed this ~2 weeks ago while printing a large part for a CNC router I was building. I came to it halfway through an 8hr print and it was only at 160C. I went to my computer and checked my temp graphs and it maintained temp for the first hour or so, then after that, it slowly tapered off. Printer stored in a ~70F room, with minimal draft.

So, Is this a sign the power supply is going (using an ATX psu, not the smaller profile, uh... newer ones)?
Maybe the heater mofset/relay (?) on the RAMBo board?
Or the hotend? Might upgrade this anyway to the newer one. No issues with the accelerometer on an older board?

Im fairly certain it is not the thermister reading incorrectly, as with the previous mentioned print, I could visually see the difference in the layer quality after that first hour of printing. And I was able to easily de-laminate it by hand.
Im leaning towards the PSU failing/not keeping up, simply because i was able to turn the bed off and the temp would go back to the target. And it seems to work fine while idle or short prints.

Doing another print right now, after maybe Ill tear it apart and swap out the PSU, as Ive got plenty of old PCs laying around.

So when you measured 160°C what id the display on your computer say it *thought* the temperature should be? At 160° I doubt that PLA would be fluid enough to extrude. Were you extruding at that point?

In rereading your post - it might be the wiring. Check that the connections are good and the wire stripped properly at the power supply to RAMBo and from RAMBo to the bed and hot end. If you have a voltmeter you can measure the voltage output at the power supply when this happens.

Yeah, ill definitely check everything over today pretty well. I do remember during setup last year, i had to lower the EXT_1_PID_Drive_Max value, so simply raising that may help. Although, im not sure what has changed in a year, besides something electrical going out.

Copper wire tarnishes, connectors vibrate loose, power supply output weakens with age/usage.

At the rate at which copper forms a patina* in a climate controlled house, i wouldnt be too worried about that. Sure, we might get a little bi-metal corrosion at the connectors, but again, in a year, thats barely any time.

Anyway, everything was good, except the hot end. One of the resistors went out/broke/idk... became super resistive.

Got a new hotend, as well as put on the accelerometer board, hopefully everything works again.