Capillary thermostats fail in a handful of predictable ways, and most can be diagnosed with a probe thermometer and a continuity tester. Work through the symptoms below in order. Safety first: isolate the equipment from the supply before opening the control panel, and let hot oil and elements cool before handling the bulb or capillary.
Symptom 1: No heat at all
Before condemning the control thermostat, check the safety limiter: a tripped manual-reset hi-limit opens the same circuit and is the most common cause of a “dead” fryer or oven. Reset it and watch whether it trips again. Then test the element and any contactor. If both are sound, check continuity across the control thermostat with the knob set above ambient temperature: no continuity means the switch mechanism has failed or the capillary system has lost its fill — replace the thermostat.
Symptom 2: Temperature drift
The dial says 175°C but a probe in the oil reads 160 or 190. Causes, in order of likelihood: the bulb is coated in carbonised oil or scale and reads slowly; the bulb has slipped out of its well or clamp so it no longer senses the medium; or the calibration has drifted after years of cycling — typical accuracy when new is ±3–5°C, and it does not improve with age. Clean and reseat the bulb first. A small, stable offset can be trimmed on models fitted with a calibration screw; an offset that is large, or that changes across the range, means the sealed system is ageing out — replace it.
Symptom 3: Contacts welded shut — runaway heat
The element never switches off, temperature climbs past the setpoint, and the hi-limit does the stopping. This is contact welding: the switch has been run at or beyond its rating, terminals have loosened and arced, or the contacts have simply reached the end of their 100,000-cycle life. Never just reset the limiter and put the equipment back in service. Replace the thermostat with the correct rating — if a 16 A unit welded on a hard-working circuit, fit the 20 A version — and check the element circuit for the overload or loose connection that helped kill it.
Symptom 4: Damaged bulb or capillary
A kinked, crushed or corroded capillary, or a dented bulb, changes the volume of the sealed liquid fill, and with it the calibration — control becomes erratic or dies completely. Damage usually happens during cleaning, tank removal or careless routing past sharp edges. There is no repair: the system cannot be refilled or re-sealed in the field. Replace the thermostat, and route the new capillary with slack, clear of edges and heat sources, exactly as the original was routed.
Symptom 5: Erratic or short cycling
The element clicks on and off every few seconds and the temperature swings. Usually the bulb is at fault, not the switch: it is touching or sitting too close to the element, half out of its well, or sensing air instead of oil or cavity temperature. Reseat the bulb fully in its well or clamp, confirm it sits in the medium it is meant to measure, and the cycling normally settles. If the bulb position is correct and cycling persists, the mechanism's differential has worn — replacement time.
When troubleshooting ends
Capillary thermostats are inexpensive, sealed and not field-serviceable beyond cleaning, reseating and a small calibration trim. When the fault is inside the unit — lost fill, welded contacts, worn mechanism — replace it, matching temperature range, current rating, SPST/SPDT contacts, capillary length and bulb size. Send us a photo of the old part's rating label and we will cross-reference the correct replacement from our capillary control thermostats range, with safety limit thermostats for the hi-limit side.