Fryer Not Holding Temperature? How to Diagnose a Failed Thermostat
What does a fryer thermostat do?
It holds the oil at the temperature you set. A capillary bulb sits in the oil, senses the temperature, and modulates the gas valve — opening it when the oil drops, closing it when the oil reaches your setting.
On a millivolt fryer, the thermostat is powered by the thermopile, not by mains electricity. That is why the fryer works in a power cut, and why a weak thermopile can make a perfectly good thermostat behave badly.
What are the symptoms of a failed thermostat?
| Symptom | What it usually means |
|---|---|
| Oil overshoots the set point — and the high limit trips | Thermostat is not shutting the gas off. The most common fault. |
| Oil never reaches temperature | Thermostat shutting off early, or a gas supply / burner problem |
| Wild temperature swings | Failing thermostat, or a capillary bulb that isn't properly immersed |
| Set to 180°C but frying like it's 200°C | Thermostat has drifted out of calibration — it can't be recalibrated, replace it |
| Fryer won't heat at all | Not the thermostat. Check the high limit (tripped?) and gas supply |
Thermostat, high limit, or gas valve — which one is it?
This trips people up constantly, so here is the clean rule:
- Fryer gets too hot → the operating thermostat has failed. It is the only part whose job is to stop the heating at your set point.
- Fryer won't heat at all → a tripped high limit, no gas, or a failed gas valve. A high limit can only ever cut heat — it can never cause overheating.
- High limit keeps tripping → the thermostat is the culprit. The high limit is catching the thermostat's failure. Replacing the high limit will not fix it.
That last one is the expensive mistake: people replace a £75–£129 high limit that was working perfectly, because it kept tripping. It kept tripping because it was doing its job.
Check these before you buy a thermostat
- Is the capillary bulb properly immersed in the oil? A bulb that has slipped out of its clip, or a low oil level, gives a false low reading — so the fryer keeps heating. This costs nothing to check and is a common "failed thermostat" that isn't.
- Is the bulb caked in carbon? A heavy carbon layer insulates the bulb from the oil. It reads cool, the fryer over-fires. Clean it.
- Is the capillary tube kinked, crushed or split? The tube is filled with fluid — damage it and the thermostat is dead. It is not repairable.
- Is the thermopile healthy? On a millivolt fryer, a weak thermopile starves the controls. Test for 250–750 mV.
If all four check out and the fryer still won't hold temperature, the thermostat has failed.
Which Robertshaw thermostat do I need?
Robertshaw RX-series millivolt thermostats are the industry standard on commercial gas fryers. Match three things:
- Temperature range — e.g. 200–400°F, 200–375°F, 250–375°F
- Fitting — 1/4 NPT or 3/8 NPT
- Capillary length — encoded in the type number (the RX-24-48 is a 48in capillary). It must reach from the control to the bulb position.
| Type | Range / fitting | Price |
|---|---|---|
| RX-19-36 | 250–375°F, 1/4 NPT | £75.00 |
| RX-1-36 | 204°C, 1/4 NPT | £80.00 |
| RX-2-24 | 200–400°F, 3/8 NPT | £80.00 |
| RX-6-36 | 200–375°F, 3/8 NPT | £80.00 |
| RX-22-36 | 200–375°F, 1/4 NPT | £80.00 |
| RX-24-48 | 185–365°C, 3/8 NPT, 48in capillary | £85.00 |
| Robertshaw Gas Thermostat | 220–400°F, 5-3/8 bulb, gland, 1/4 NPT | £149.00 |
The type number is printed on the body of the old thermostat. Quote it and you get an exact match. If it is unreadable, send us a photo along with your fryer make and model.
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Fitting
The capillary tube is the fragile part. It must be routed without kinks and the bulb seated in its clip at the correct depth in the oil. Kink the capillary on fitting and the new thermostat is scrap.
UK commercial catering gas work must be carried out by a Gas Safe registered engineer qualified for commercial catering.
Frequently asked questions
Can a fryer thermostat be recalibrated?
No. Once it has drifted, it is replaced. If your fryer is consistently frying hotter or cooler than the dial says, the thermostat is telling you it is finished.
My high limit keeps tripping — should I replace the high limit?
Almost certainly not. The high limit is tripping because the oil is genuinely overheating, which means the thermostat is not shutting the gas off. Replace the thermostat. See our high limit guide.
What does "millivolt" mean on a fryer thermostat?
It means the thermostat is powered by the thermopile's own output (250–750 mV) rather than mains electricity. It is a self-powered control system — which is why the fryer runs without being plugged in.
My fryer runs hot only when it's busy. Why?
Check the bulb is properly immersed and the oil level is correct. During heavy service oil level drops, and a partly exposed bulb reads cool and keeps calling for heat.
How fast can I get one?
UK stock from £75. Order before 10am for next-day delivery. Call 073 8051 5838 with the type number from the old unit.
Get the right thermostat
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