Auxiliary contact not making — seal-in or interlock fails
The contactor pulls in, but an auxiliary contact (used for seal-in, interlock, or status) doesn't change state, so the circuit won't latch, an interlock misbehaves, or status feedback is wrong.
Safety first
Auxiliary contacts often handle interlocks and seal-in. A faulty one can let a circuit behave unexpectedly — confirm the sequence is safe before testing repeatedly.
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
Worn or dirty auxiliary contact
Most likelyAux contacts carry small currents and can develop a film or wear that stops a reliable make.
- 2
Wrong contact type used (NO vs NC)
#2The circuit expects a normally-open make but is landed on a normally-closed contact (or vice versa).
- 3
Loose terminal on the aux block
#3A loose wire at the auxiliary block breaks the seal-in/interlock path even though the contact itself is fine.
- 4
Add-on aux block not engaging the contactor
Least likelyA clip-on auxiliary block that isn't seated properly won't be driven by the contactor's mechanism.
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.
With the contactor energised, measure across the auxiliary contact to confirm it changes state.
The aux contact makes (or breaks) as expected when the contactor pulls in.
Aux contact works — the fault is in the wiring landed on it.
The aux contact isn't switching — check its type, seating, and condition.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Aux contact not making
→ step 2 - 2decision
Does the aux contact change state when the contactor pulls in?
Yes→ step 3No→ step 4 - 3result
Contact works — the fault is in the wiring landed on it.
- 4decision
Is it the correct NO/NC contact type, seated and tight?
Yes→ step 5No→ step 6 - 5result
Mechanically correct but not switching — replace the aux block.
- 6result
Move to the correct contact / reseat block / tighten terminals.
Common mistakes apprentices make
- Confusing NO and NC auxiliary contacts when wiring a seal-in.
- Blaming the main contactor when only the aux block is at fault.
- Missing a clip-on aux block that isn't fully engaged.
- Not checking terminal tightness on the small aux wires.
When to stop & escalate
If the aux contact forms part of an interlock that protects equipment or people, confirm the intended logic against the drawings before changing anything.
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
Contactor has voltage at the coil but won't pull in
You measure the rated control voltage (e.g. 24V) across the coil terminals, but the contactor refuses to energise — no clunk, no pull-in, contacts stay open.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
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.
Learn the theory
How the gear and circuits behind this fault actually work.