AdvancedHigh risk

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.

Start fault-finding

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.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
SSR failed short-circuit
Most likely
SSRs commonly fail in the conducting (shorted) state, so the load stays powered regardless of the control input.
2
Control signal still present
#2
The 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
#3
Some 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 likely
An 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.

Premium fault tree

The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 3

Command the load OFF and measure the SSR control (input) signal.

Expected reading

Control input goes to its 'off' state when the controller commands off.

Your actual reading

If it passes

Control says off but the load stays on — the SSR has very likely failed short.

If it fails

The control signal is still present — the SSR is being told to stay on. Chase the controller/wiring.

Often leads toHeater bank drawing uneven current across phases PLC output LED is on but the device doesn't work

View all expected readings at once

1. Command the load OFF and measure the SSR control (input) signal.

Control input goes to its 'off' state when the controller commands off.

2. With control confirmed off, isolate, lock off, and check the SSR output across its load terminals (continuity/resistance) per your procedure.

A high resistance / open across the output when off.

3. After replacing, check the SSR's heatsinking and that it's correctly rated for the load.

Adequate heatsink and an SSR rating comfortably above the load.

Full test sequence

The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Fault-finding flowchart

The same logic as a decision tree.

1
start
SSR stuck on
→ step 2
2
decision
With OFF commanded, is the SSR control input actually off?
Yes→ step 3No→ step 4
3
decision
Does the SSR output block (high resistance) when off?
Yes→ step 5No→ step 6
4
result
Controller/wiring still commanding ON — fix the control signal.
5
result
SSR blocks correctly — investigate leakage or a sensitive load.
6
result
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.

Related faults

Heating elements & SSRs

Heater bank drawing uneven current across phases

A multi-element heater bank pulls noticeably different current on each phase. Heating is uneven, output is low, or a phase reads much lower than the others.

5 causes4 test steps

QualifiedMedium risk

PLC inputs / outputs

PLC output LED is on but the device doesn't work

The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.

5 causes4 test steps

QualifiedMedium risk

Timers

Timer relay not switching its output

A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.

5 causes3 test steps

ApprenticeLow risk

Learn the theory

How the gear and circuits behind this fault actually work.

Device

SSR (Solid-State Relay)

Switches a load electronically with no moving parts — fast, silent, ideal for frequent switching like heaters.