The symptom pattern

The printer is asked to start a print, the hotend begins to warm, and within half a minute the display stops everything with Preheat error print head and the code #12202. Nothing has actually printed. Power-cycling gets the same result, sometimes after the temperature has crept up a few degrees, sometimes with the reading barely moving off ambient.

It is easy to read this as a dead mainboard, because the printer appears to be refusing to do anything at all. It is more often a wiring fault a few centimetres from the nozzle.

What’s actually happening

Prusa’s firmware raises #12202 when the printer cannot heat the hotend more than 2 °C in 20 seconds. That is a deliberately specific test, and it is worth understanding what it rules out. The check does not say the hotend got hot and then lost heat — that is the separate thermal runaway error, #12204, which fires when the temperature drops and does not recover. #12202 says the heat never arrived in the first place.

Two things produce that. Either the heater is not receiving power, or the thermistor is not reporting the temperature it should. Both are usually a connection problem rather than a dead part, because the print head is the one place on the machine where the cables are flexed thousands of times per print. A conductor can fracture inside insulation that still looks perfect.

Working through it

Start with your eyes, with the printer off and unplugged. Follow the loom from the head back and look for split insulation or a crushed section. Then move the wires by hand near the head — intermittent faults often only reveal themselves under movement, and that is a much faster diagnosis than reading temperatures.

Next, open the Buddy board’s cable cover and check the thermistor and heater connectors are properly seated. Connectors that have vibrated part-way out of their sockets are common enough to be worth eliminating before reaching for the multimeter.

Then measure — printer off, at room temperature, and the component unplugged from the mainboard. On the 200 kΩ range the thermistor should read 80 kΩ to 125 kΩ; Prusa rates its thermistors at 100 kΩ at 25 °C and gives that band to cover normal room temperatures. The heater goes on the 200 Ω range: Prusa’s published figure for the MINI’s hot-end heater is 12.3 Ω to 15.1 Ω — the same window Prusa quotes for the MK3, MK4, XL and CORE One. A reading inside that band is normal; an open circuit identifies the failed side outright.

The step people skip is the useful one: take both readings again while flexing the cable where it enters the head. A component that measures correctly at rest but drops out when moved is not a faulty component — it is a broken conductor, and swapping the part will not fix it.

One thing not to do if you find a broken thermistor wire: Prusa specifically advises against soldering these back, because the joint changes the thermistor’s resistance and therefore the temperature the printer thinks it is reading.

Verdict

Worth repairing, and cheaply. The thermistor and heater cartridge are both inexpensive service parts sold individually, with Prusa service guides for fitting them, so once the meter has told you which side has failed the job is straightforward. The important thing is to let the flex test decide between a failed part and a failed cable, because those are two different repairs and only one of them is fixed by buying a new heater.

Isolate the printer at the mains before opening anything or taking any reading — these are low-voltage measurements, but the PSU behind them is not.