Solid-state relay (SSR) stuck on — heat won't switch off
A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.
Safety first
A stuck-on SSR means the load can stay energised with no control. Don't rely on the controller to make it safe — isolate upstream. Runaway heating is a burn and fire risk; treat it as urgent.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
SSR failed short-circuit
Most likelySSRs commonly fail in the conducting (shorted) state, so the load stays powered regardless of the control input.
- 2
Control signal still present
#2The controller is still commanding the SSR on (stuck output, wrong logic, or a control wiring fault), so the SSR is doing exactly what it's told.
- 3
Wrong SSR type or leakage misread
#3Some SSRs have small leakage current when 'off'; a sensitive load or a misread can look like 'stuck on' when it isn't fully conducting.
- 4
Overheated / under-rated SSR
Least likelyAn SSR run beyond its rating or with poor heatsinking can degrade and fail short.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeCommand the load OFF and measure the SSR control (input) signal.
Control input goes to its 'off' state when the controller commands off.
Control says off but the load stays on — the SSR has very likely failed short.
The control signal is still present — the SSR is being told to stay on. Chase the controller/wiring.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
SSR stuck on
→ step 2 - 2decision
With OFF commanded, is the SSR control input actually off?
Yes→ step 3No→ step 4 - 3decision
Does the SSR output block (high resistance) when off?
Yes→ step 5No→ step 6 - 4result
Controller/wiring still commanding ON — fix the control signal.
- 5result
SSR blocks correctly — investigate leakage or a sensitive load.
- 6result
SSR failed short — replace, then check heatsink/rating.
Common mistakes apprentices make
- Trusting the controller to switch the heat off in a fault — a shorted SSR ignores it entirely.
- Replacing the SSR but reusing a poor heatsink or under-rated device, causing a repeat failure.
- Mistaking normal SSR leakage for 'stuck on'.
- Not isolating upstream before working, leaving the load live.
When to stop & escalate
A heating circuit that can run away needs an independent over-temperature safeguard — if there isn't one, raise it. Repeated SSR failures point to a sizing/cooling design issue that should be reviewed, not just re-fitted.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Learn the theory
How the gear and circuits behind this fault actually work.