Air compressor won't start or keeps cutting out
The workshop compressor hums without starting, trips its overload or the breaker, or starts and cuts out before reaching pressure. Nearly always one of four things: start capacitor, pressure switch/unloader, head pressure it can't start against, or a supply that sags.
Safety first
Two stored energies here: capacitors hold charge (discharge safely before testing) and the tank holds air — never crack fittings on a pressurised tank, and isolate electrically before touching anything. Vent per the machine's procedure.
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
Failed start capacitor (single-phase units)
Most likelyThe classic hum-no-start. The cap dries out or bulges, the motor can't develop starting torque, and the overload takes it out a few seconds later.
- 2
Unloader valve not venting the head
#2If the little hiss after each stop is missing, the pump stays pressurised — and the motor is asked to start against a wall. Hums, strains, trips.
- 3
Pressure switch faulty or badly set
#3Burnt contacts never call for a start; a mis-set or drifted switch short-cycles the machine or never cuts back in.
- 4
Supply voltage sagging under starting load
#4The long skinny extension lead special. Starting current through a thin lead drops the voltage exactly when the motor needs it most — hums, trips, and everyone blames the compressor.
- 5
Motor or pump mechanically tight
Least likelySeized pump bearings, tight rings after a rebuild, or a failing motor bearing raise the starting torque past what's available.
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 ModeVent the tank (per procedure) and try a start from EMPTY. Listen for the unloader's hiss when it stops.
Starts happily empty: the problem is starting under pressure — unloader or non-return valve. No hiss after stopping backs that up.
Chase the unloader/NRV on the pneumatic side — the electrics are likely fine.
Won't start even empty: it's electrical or mechanical — capacitor first.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Compressor won't start / cuts out
→ step 2 - 2decision
Does it start fine from an EMPTY tank?
Yes→ step 3No→ step 4 - 3result
Starting against head pressure — unloader valve / non-return valve on the pneumatic side.
- 4decision
Does the start capacitor test at its rated µF?
Yes→ step 5No→ step 6 - 5decision
Does the voltage at the machine hold up during the start attempt?
Yes→ step 7No→ step 8 - 6result
Replace the capacitor like-for-like (µF and voltage).
- 7result
Check the pressure switch, then bar the pump over — tight mechanicals or a tired motor from here.
- 8result
Supply sag — proper lead or a correctly sized fixed circuit.
Common mistakes apprentices make
- Replacing the capacitor without checking the unloader — the new cap dies the same death starting against head pressure.
- Testing supply voltage at the wall while the compressor starves at the end of 30 m of bargain extension lead.
- Winding the pressure switch up to 'fix' short cycling and cooking the motor instead.
- Cracking fittings on a tank that's still holding pressure.
When to stop & escalate
Pump rebuilds, ring/bearing work and anything on the pressure vessel itself belong with a compressor mechanic — and pressure vessels have their own inspection rules on industrial sites. A motor drawing high with everything else proven good is a rewind-or-replace economics decision.
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
Motor hums but won't start to turn
On start the motor hums or buzzes but won't rotate — it may draw heavy current and trip protection. Often a sign it's only getting two phases or is mechanically locked.
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
Equipment tripping the supply on startup surge
Switching on a piece of equipment trips an upstream breaker or causes a momentary dip — the inrush/startup surge is exceeding what the protection or supply can ride through.