Changeback faults
Generator Not Switching Back to Mains
When the mains comes back, your transfer switch should move the building back onto the mains, let the generator cool down and then stop it. If the building stays on generator power, or the switch keeps flicking between the two, the fault is almost always in the mains sensing, the transfer switch or its timers rather than in the engine. Leave the board closed and call us; we test it with the supply made safe.
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- Mains sensing and timers tested
- Board left closed for us
- Open 24/7 for transfer faults
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The return half of the sequence
Why a generator does not switch back to the mains
Changing back is a separate step from starting. The transfer switch has to see that the mains has returned and is stable, wait for its return delay, move the load across, and then signal the generator to cool down and stop. A fault at any of those steps leaves the building on generator power.
Some of this is normal. Many systems wait a few minutes to make sure the mains is stable, and then run the engine for a cool-down period before stopping it. If the building has switched back and the engine keeps running for a short while, that may be by design.
If you have a manual changeover, nothing switches back by itself: someone has to move the switch back to mains before stopping the generator. Our changeover switch page explains both types.
Fault guide
Generator not switching back to mains: what it can mean and what to do
When the mains returns, the transfer switch should move the load back and let the generator cool down and stop. If the building stays on generator power, the fault is usually in the mains sensing, the transfer switch or its timers, not the engine. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| The mains is back but the set keeps supplying | The transfer switch is not sensing the mains return | ATS repairs |
| Switches back, but the set keeps running | Often a normal cool-down run; otherwise a stop signal fault | Controller repairs |
| A manual changeover is still on the generator side | A manual switch has to be moved back by hand | Changeover switch repairs |
| Switches back and forth repeatedly | Unstable mains, or sensing limits and timers set too tight | ATS repairs |
01
What you may notice
- The mains is back but the building is still on the generator
- The generator keeps running long after the mains returned
- The switch moves back and forth between mains and generator
- A manual changeover is still in the generator position
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- The transfer switch is not sensing that the mains has returned
- A return delay or cool-down timer set longer than expected
- A contactor or motorised switch that does not move back
- Mains voltage outside the limits the switch accepts
- A stop signal fault between the switch and the controller
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- Do neighbours have power, and do your mains lights or meter show power?
- How long after the mains returned has the generator kept supplying?
- What does the controller screen show? A photo helps.
- Is your changeover manual or automatic?
04
Stop using the generator and call if you notice
- Buzzing, heat or a burning smell from the changeover or board
- Lights flickering as the switch moves back and forth
- Any sign of both supplies being connected at once
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Mains sensing inputs and the voltage limits set in the switch
- Return delay and cool-down timer settings
- The contactor or motorised mechanism and its control wiring
- The stop signal between the transfer switch and the controller
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- Whether the changeover is manual or automatic
- A photo of the controller screen
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Step 9: book a diagnosis visit.
Related Pages
Transfer switches and controllers
- ATS Repairs Automatic transfer switch faults
- Changeover Switches Manual changeovers and the CoC
- Not Starting Automatically The first half of the sequence
- Controller Repairs Stop signals and settings
- Generator Controllers Reading the controller screen
- Deep Sea (DSE) Controllers Return delay and cooling run
- Emergency Repairs 24/7 callouts
Common Questions
Questions about switching back to mains
Why has my generator not switched back to mains?
Usually the transfer switch has not sensed the mains return, a return delay is set longer than expected, or the contactor or switch mechanism has not moved. We test each one with the supply made safe.
The building is back on mains but the generator is still running. Is that a fault?
Not always. Many systems run the engine for a cool-down period after switching back. If it keeps running well beyond a few minutes, the stop signal or the controller needs checking.
Why does my generator keep switching between mains and generator?
An unstable mains supply, or voltage limits and timers set too tightly, can make the switch move back and forth. Repeated switching wears the contactor, so call us.
Can I switch it back myself?
Only on a manual changeover, by moving the switch back to mains before stopping the generator. Never force an automatic switch or open the board; parts are live.
Is this the same as the generator not starting automatically?
No. That is the first half of the sequence. Our guide to a generator not starting automatically covers it.
Do you work on transfer switches after hours?
Yes. We are open 24/7, including public holidays, and we arrive within 1 hour when a transfer fault leaves you without normal power.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Tell us what the switch is doing
Is the changeover manual or automatic, and is the building still on generator power? A photo of the controller screen helps.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.