DOL starter won't latch when you press start
Press start on a direct-on-line starter and nothing latches — the contactor doesn't pull in at all, or pulls in and immediately drops. The motor never runs.
Safety first
You'll be testing a live control circuit beside live motor terminals. Confirm what's isolated, follow the starter drawing, and don't bridge out the stop, overload, or any safety contact to make it start.
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
Tripped or open overload contact
Most likelyThe overload's auxiliary (normally-closed) contact is open from a previous trip or a fault, breaking the control circuit so the contactor can't energise.
- 2
Stop button / E-stop open
#2A latched E-stop, an open stop button, or a control isolator left off keeps the start circuit broken.
- 3
No control voltage to the start circuit
#3A blown control fuse, failed control transformer, or lost supply means pressing start does nothing.
- 4
Faulty start button or seal-in
Least likelyA start button not making, or a seal-in auxiliary not holding, prevents the latch even though the rest of the circuit is healthy.
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 ModeConfirm control voltage is present at the start of the control circuit.
Rated control voltage available to the start circuit.
Control supply is there — work along the series contacts toward the coil.
No control voltage — fix the control supply (fuse/transformer/isolator) first.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
DOL starter won't latch
→ step 2 - 2decision
Is control voltage present at the start circuit?
Yes→ step 3No→ step 4 - 3decision
Are overload and stop/E-stop contacts closed (reset)?
Yes→ step 5No→ step 6 - 4result
No control supply — fix fuse/transformer/control isolator.
- 5decision
Does the start button make and the seal-in hold?
Yes→ step 7No→ step 8 - 6result
Tripped overload or open stop/E-stop — clear it (after finding the cause).
- 7result
Control path is good — recheck the contactor coil and its feed.
- 8result
Faulty start button or seal-in — repair the button/seal-in path.
Common mistakes apprentices make
- Bridging out the overload or stop to 'test' and leaving it like that.
- Not checking the overload tripped earlier and is still latched open.
- Assuming the contactor coil is dead before confirming control voltage.
- Missing a control isolator or E-stop left in the off/latched position.
When to stop & escalate
If the overload tripped, find why before resetting and running again. If a safety contact (E-stop/guard) is open, treat that as the priority and never bypass it to start the motor.
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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Related faults
Contactor pulls in on the button but won't seal in
Press start and the contactor pulls in, but it drops out the moment you release the button. It won't latch/hold itself in through the seal-in circuit.
Overload relay won't reset
After an overload trip, the reset won't take — the starter won't re-arm, or it resets and trips straight back. The control circuit stays broken at the overload's contact.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.