Intermittent MAX TEMP Error

[DaIceMan]

Hi all, after several successful prints in the last months, I started getting more and more MAX TEMP errors (extruder). The temperature starts rising at a certain point and reaches the max temp threshold and the printing shuts down. It is not a connection issue as the cables are fine (no shorts on the thermistor). Moving the Thermistor wires near the head does not produce any jumpy readings when the printer is working normally. It appears that at a certain point the temp control goes "runaway" and keeps heating for no apparent reason. Sometimes I get this error at heat up, before the actual printing. The reported temperature appears correct, just the control seems flaky. Does anyone have any suggestions on what to test before I replace any part? The printer is a few months old (I built it from kit) and initially worked perfectly. If the thermistor were broken it would not read or give out aberrant readings but what happens is just that the heating element stays on even after the target temperature has been reached ( I saw this once before the MAX_TEMP error) so I really can't imagine what could be the issue.
Thanks for any feedback.

[DaIceMan]

Ok I found the problem: the extruder temp control FET was intermittently "shorting". I found out by touching the heatsink and moving it slightly. It would turn on and off (indicated by the relevant RED LED on it's Drain output). I also found out that the second Heat FET did the same, but nothing was connected to it so I never realized the problem as I only have one head. I unsoldered the FET and removed it from the pcb. The FET was fine but the heatsink (which is connected to the Drain) was evidently touching the PCB and triggering the gate (FETs have a very high impedance and usually the gate should be pulled down or up to avoid uncontrolled conduction). I removed the central PIN from the heatsink for safety and isolated the bottom of the heatsink with a small piece of adhesive paper (a paper sticker will do). I did this also for the other 2 FETs. Once mounted again, the board was working fine again, even while pushing around the heatsink the LEDs stayed off and the temp control was finally working fine again (LED flashes reaching the target temp and keeps it stable). So if any of you have intermittend temp control problems with the GT2560 board, CHECK YOUR FET HEATSINK and isolate it.

[William]

Got it.Thanks for sharing