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I have a V2, just upgraded to the standard 12V power supply from the old ATX psu but I'm still having issues with heating. I ran the PID tuning and when I tried to preheat to 240, the temperature stopped climbing around 228 and I got the decoupled error message.

At what temperature was the tuning started, and where was it run to? Different set points want different PID's (And the Rambo's autotune model, while nice enough, has some rough spots with that).

Assuming you are still using the original hotend, I would check the condition of the resistors in the hotend.

Xenocrates wrote:At what temperature was the tuning started, and where was it run to? Different set points want different PID's (And the Rambo's autotune model, while nice enough, has some rough spots with that).

PID was run at 200

That's a bit low from my perspective. PID on a rambo works best around the operating temperature it's tuned to. If you picked 220-230, that may work a lot better than tuning at 200. Tuning at 240 would be best, but with the original PEEK based hotends, the potential for overshot damaging the hotend is a bit high for my taste. You could try increasing the drive max or the P term, if you don't want to re-do the auto-tuning. And as Mac mentioned, the resistors in the hotend, especially if they are the carbon-composition resistors with a silicone potting, may have degraded or failed, possibly due to age, overheating, mechanical strain, or a contaminant in the potting compound, such as air, water, ETC. So you'll want to check them with a meter, and if they have gone bad, my suggestion is to buy an M3 tap, some grub screws, and a heater cartridge. Geneb has a nice tutorial on installing them.