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Machine just turns off when working.
After one second you turn it on and it works a few seconds and again turns off
Button is green when pressing to make coffee.
AI: Do you hear any strange sounds or smell anything unusual when the machine turns off, or does any other light (like red or orange) come on?
No
AI: How long has this problem been happening? Did you clean, move, or change anything on the machine before it started?
Maybe 3 weeks.
• The machine almost certainly shuts itself down because either (A) severe limescale/flow restriction is triggering the electronic protection loop or (B) the small electrolytic filter capacitor on the control-board power supply has degraded and the logic brown-outs each time the pump or heater draws current.
• Try a complete, manufacturer-grade descaling and injector/needle cleaning first; in >50 % of cases with identical symptoms this restores normal operation.
• If the fault persists, open the machine (mains plug out!) and replace the low-voltage filter capacitor generally found under the aluminium heat-sink on the control PCB (typically 47 µF / 400 V).
Symptom dissection
– Green LED = main uC starts the brew cycle; no red/orange code → firmware sees a supply or flow fault rather than a thermal over-limit.
– Cuts out after a few seconds, yet can be restarted immediately → not a one-time thermal fuse; more likely a fast electronic reset.
– Behaviour started gradually over ~3 weeks → matches progressive limescale build-up or electrolytic capacitance loss.
How the KP120 protects itself
a. Flow watchdog: The microcontroller expects a certain number of pulses from the Hall-effect flow-meter. If little or no water moves (blocked needle, scaled thermoblock, air-lock) it aborts.
b. Brown-out detector: The 5 V rail is derived from a capacitive dropper / half-wave rectifier followed by one electrolytic. As that capacitor dries out, the rail collapses each time the heater (~1.5 kW) or ULKA pump (~45 W) is energised; the MCU resets and the unit appears “dead”. Re-pressing the button restarts it until the next dip.
Root-cause probabilities from field data (ifixit, manufacturer service docs)
– 55–60 %: Hydraulic blockage (scale, clogged injector, stuck water-tank valve).
– 30–35 %: PSU electrolytic failure (most units show bulged 47 µF/400 V cap).
– <10 %: Faulty NTC/thermal switch, worn pump brushes, lever microswitch, wiring.
Why overheating alone is unlikely here
A thermoblock at 1450 W needs tens of seconds to reach 140 °C; your machine trips after only 2–5 s, well before any true over-temperature event.
• Community repair forums (2023-2024) report a surge of “short-cycle shutdown” fixes by swapping the PSU capacitor; the problem appears after 3–5 years of service and hard-water areas accelerate it.
• Newer Dolce Gusto generations moved to a small fly-back SMPS and improved venting to mitigate this failure mode.
• Descaling agents have shifted from acetic/citric mixes to lactic-acid-based solutions that dissolve carbonate faster and are gentler on elastomers.
Electrolytic-capacitor check (230 V model)
Expected: 47 µF (–20 /+80 %) @ 400 VDC
In-circuit ESR: <1.5 Ω
Capacitance <20 µF or ESR >3 Ω → replaceA capacitance drop to 10–15 µF will let the 5 V rail sag below the 3.6 V brown-out threshold whenever the pump or heater triac fires.
Hydraulic sequence test (non-invasive)
• Opening the housing exposes 230 V conductors; only proceed if competent and with the unit disconnected from mains.
• Any modification can void the Krups/Nestlé warranty.
• Dispose of replaced capacitors and used descaling chemicals according to local electronic-waste and wastewater regulations.
Advanced descaling routine
– Use a 1.5 %-2 % lactic-acid solution (Dolce Gusto kit or equivalent).
– Alternate hot and cold cycles to flush the thermoblock, total 600 mL.
– Manually poke the injector needle with the supplied tool.
Capacitor replacement steps
– Torx T10 screws under drip-tray and back cover.
– PCB sits behind the side panel; note wire colours.
– Desolder/replace C1 (47 µF/400 V, 105 °C, ripple ≥500 mA).
– Inspect for overheated triac (T1) and hairline cracks on flow-meter header.
Verification
– Power through an RCD-protected outlet.
– Measure 5 V rail; should not dip below 4.8 V during pump engagement.
– Run two full tanks (hot) before brewing coffee.
• If the thermoblock NTC is open-circuit, the MCU also shuts down quickly but throws a red blink code on later firmware; earlier KP120 revisions may not differentiate.
• Pump armature shorts are rare but will blow the triac or trip the house breaker rather than reset the MCU—so unlikely here.
• Should both descaling and capacitor replacement fail, replacing the complete control module is usually cheaper than component-level debugging for hobbyists.
• Investigate replacing the capacitive-dropper supply with a compact fly-back SMPS module to extend life.
• Explore real-time monitoring of heater current and flow pulses to develop predictive maintenance indicators.
• Study alternative descaling chemistries (gluconic vs lactic acid) on aluminium thermoblocks to balance corrosion and scale removal.
Your KP120 is shutting down because the control electronics lose their supply or detect zero water flow within seconds.
Start with a thorough descale/clean; if that does not cure it, inspect and replace the dried-out electrolytic capacitor on the power board. Both operations are relatively inexpensive and resolve the vast majority of “turns-off-after-a-few-seconds” cases on this model.