Float / level switch not operating
A float or level switch isn't controlling correctly — a pump won't start/stop on level, a tank overfills or runs dry, because the level switch isn't changing state.
Safety first
A failed level control can overfill (flood) or run a pump dry (damage). Confirm safe levels manually before relying on the switch. Isolate before working on wiring.
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
Float mechanically stuck / fouled
Most likelyDebris, scale, or a tangled float cable stops the float moving with the level.
- 2
Float switch contact failed
#2The internal contact has failed open or closed and no longer switches.
- 3
Wiring / connection fault
#3A broken cable or loose terminal between the float and the control breaks the signal.
- 4
Wrong float type / orientation
Least likelyA float fitted upside-down or the wrong type switches the wrong way.
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.
Move the float through its range by hand and listen/feel for the switch action.
A clear switch action as the float tilts/rises and falls.
It switches by hand — check wiring and orientation.
No action / stuck float — free it or test the contact.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Float/level switch not operating
→ step 2 - 2decision
Does it switch when you move the float by hand?
Yes→ step 3No→ step 4 - 3decision
Is the wiring sound and float orientation/type correct?
Yes→ step 5No→ step 6 - 4decision
Does the contact change state on a continuity check?
Yes→ step 7No→ step 8 - 5result
Re-verify the control logic.
- 6result
Wiring fault or wrong orientation — correct it.
- 7result
Contact works — fault is in wiring/control side.
- 8result
Stuck float or failed contact — free it or replace the switch.
Common mistakes apprentices make
- Not checking the float is physically free to move.
- Fitting a float upside-down so it switches the wrong way.
- Blaming the control when the float contact has failed.
- Relying on the switch before confirming safe levels manually.
When to stop & escalate
Where overfill/dry-run could cause damage or a hazard, ensure an independent high/low safeguard exists. Persistent fouling points to a maintenance/access issue to address.
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
Limit switch or proximity sensor not being detected
A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.
Analogue sensor (4-20mA / 0-10V) reading wrong
An analogue sensor (pressure, level, temperature) gives a wrong or fixed reading — stuck at zero, pinned at full scale, or simply not matching reality.
PLC input not reading despite a signal present
A field device is clearly providing a signal, but the PLC input bit/LED doesn't come on — the program never sees the input, so logic that depends on it won't run.