I get this call a lot — engine sounds great, fuel system is fine, the unit purrs the way it should. They plug in a light and get nothing. This is a completely different beast than a generator that will not start, and I always relax a little when I hear this symptom because the diagnostic path is shorter and the parts are cheaper. I spent nine years repairing commercial food service equipment — motors, windings, capacitors, control boards — and the failure modes I see on portable generators are the same ones I chased in supermarket back rooms. Predictable, testable, almost always fixable.
The 30-second answer: When a generator runs but no power reaches the outlets, I check four suspects in this order: the breaker on the generator panel (always first, often the answer), lost residual magnetism in the rotor (free fix with a 12V battery), a failed capacitor on brushless units, then a failed AVR or worn brushes. Most fixes are $0-60 in parts. The 12V flash-the-field procedure restores about a third of these cases with no parts at all.
Why I always relax a little on this symptom
When the engine is running but the outlets are dead, I already know three things. The engine is fine. The fuel system is fine. Spark and compression are fine. Everything I just ruled out is the hard, dirty half of small-engine work. What I have left is the alternator end — four parts, four tests, four fixes. Let me walk through each one the way I would teach an apprentice who had never opened up the back of a generator before.
IMAGE_NEEDED: Photo of a generator control panel with the output breaker visible and ready to reset, captioned “Always my first stop. The breaker trips more often than people think and the fix is free.”
Suspect #1 — The output breaker (I check this first, every time)
How it works
Every portable generator has at least one breaker on its output panel — usually a 20A or 30A push-button or toggle. It protects the alternator windings from a downstream short. If it trips, the engine keeps running but nothing reaches the outlets. Exactly the symptom I am diagnosing.
How it fails
It does not really fail — it does its job. I have seen this trip from too-big a load (1500W heater on a 1200W unit), a shorted extension cord, or a damaged appliance. Most people never look at the breaker before assuming the alternator is dead.
How I test and reset
I look at the panel. Most breakers either show a red flag, a midway-pushed handle, or have an obvious tripped position between ON and OFF. I push to OFF firmly, then back to ON. Done.
How to replace
If the breaker pops again immediately under no load, the breaker itself has failed — $8-15, two screw terminals. If it pops only when I plug something in, I have a real downstream problem (cord, plug, or appliance) or the breaker is undersized for the load. See generator tripping breaker for that diagnostic path.
Suspect #2 — Lost residual magnetism (the free fix most people skip)
How it works
This is the one most homeowners have never heard of, and I find it is the answer about a third of the time on stored generators. Every brushless portable depends on a tiny amount of permanent magnetism baked into the rotor steel to “bootstrap” its own voltage at startup. That residual field, spinning past the stator windings, induces a small voltage. The AVR sees that small voltage and dumps current back into the rotor field to amplify it — and that is how the generator self-excites up to full output. Without that initial residual magnetism, the AVR has nothing to amplify. Engine spins fine, output reads zero.
How it fails
I see residual magnetism decay in three situations: the unit sat unused for many months, the unit was shut down under a heavy load (which can reverse-magnetize the rotor briefly), or a severe short hit the alternator. After a long winter in the shed, plenty of generators wake up with their residual magnetism gone — and the owner thinks the alternator is dead when the fix is free.
The 12V flash — my favorite free fix
I disconnect the wires going to the brush holder, or to the rotor field connections on a brushless unit (wiring varies — I always have the manual or a wiring diagram in front of me). I start the engine and let it run at normal speed. Then I take a 12V car or motorcycle battery and briefly — 2-3 seconds max — touch the positive lead to the field positive wire and the negative lead to the ground side. That brief DC pulse re-magnetizes the rotor. If lost residual magnetism was the problem, output appears on the panel meter or at the outlets immediately. I disconnect the battery, reconnect the AVR, and the unit self-excites normally from then on.
For my full step-by-step procedure with wiring color codes for common brands, see the AVR guides. Engineer Fix has a detailed walkthrough if you want a second source.
IMAGE_NEEDED: Diagram or photo of the 12V flash setup: battery, AVR disconnected, leads to field wires, captioned “The free fix. Three seconds of DC and residual magnetism is back.”
Suspect #3 — The capacitor (cheap, common, easy to test)
How it works
Most inexpensive brushless portable generators use a capacitor instead of (or alongside) an AVR to provide the excitation current to the rotor. The cap sits on the alternator end, about the size of a beer can, rated at a specific capacitance value (typically 20-40 µF) at the operating voltage of the alternator.
How it fails
I see capacitor failures from two causes: age (electrolytic caps have a 5-10 year design life), and heat (mounting near a hot engine cooks them faster). When one dies, it usually loses capacitance gradually — weak output before total death. A bulging end cap or any sign of leakage is a dead giveaway and I do not even bother testing at that point.
How I test it
I always discharge it first — short the terminals through a screwdriver with an insulated handle. A charged cap will bite hard. Then I disconnect both leads and read with my multimeter on the capacitance setting. The reading should be within about 10% of the value printed on the cap. A 25 µF cap should read 22.5-27.5 µF. Way off, or zero, means it is dead.
How to replace
I match the capacitance value (µF) and the voltage rating. Replacements run $10-25 and are usually two-terminal — disconnect, swap, reconnect. Five-minute job. After replacement, the unit should produce full output immediately if the cap was the only problem.
| Part | DIY price | Test tool | Time to swap |
|---|---|---|---|
| Output breaker (panel) | $8-15 | Visual | 10 min |
| Capacitor (typical 20-40 µF) | $10-25 | Multimeter on µF | 5-10 min |
| AVR (model-specific) | $20-60 | Multimeter DC volts | 30-45 min |
| Carbon brushes (where fitted) | $10-20 | Visual + continuity | 20-30 min |
Suspect #4 — The AVR (the brain of the alternator)
How it works
An AVR — automatic voltage regulator — is a small encapsulated electronics board that monitors output voltage and adjusts the current flowing into the rotor field windings to keep output at the rated voltage (120V or 240V on most U.S. portables). On brushless generators with AVRs, this is the active brain. When one fails, I see output that is zero, way too low, or wildly fluctuating depending on how the failure happened.
How it fails
Heat and voltage spikes are the killers. A breaker trip under heavy load can send a back-EMF spike into the AVR. Running at full rated output for many hours in hot weather overheats the regulator’s MOSFETs. On older portables (10+ years) I see them fail from heat cycling regardless of how lightly the unit was used.
How I test it
I test in this order: input voltage from the alternator excitation winding (varies by brand, usually 100-200V AC at the sense terminals while running), then output voltage to the field (should be 30-70V DC under normal operation). Numbers way off the manual spec mean the AVR is not doing its job. Honda inverters use a more complex inverter board that fails differently and is usually replaced as a unit.
How to replace
I locate the AVR — usually a small encapsulated board mounted to the alternator end. I photograph the wire colors and positions before I disconnect anything. Swap, reconnect, test. Brand-matched AVRs run $20-60 on Amazon. Some generic universal AVRs work across multiple brands, but I prefer matching the original where I can.
Suspect #5 — Carbon brushes (brushed units only)
How they work
Older portables and some heavy-duty units use carbon brushes that ride on slip rings to transfer DC excitation current into the spinning rotor. As they wear, contact gets intermittent.
How they fail
Wear from use. A typical set lasts hundreds of running hours. Inverter generators have no brushes at all (they use permanent magnets in the rotor).
How I check and replace
I pull the brush holder cap on the back of the alternator. The brushes are spring-loaded carbon rectangles, about half an inch long when new. I replace at one-quarter inch or less. New brushes are $10-20 and slide right in. Five-minute job once I find the access cover.
Safety — high voltage and stored energy.
I always shut the generator off and let it sit for a few minutes before opening any panel — capacitors hold a charge that will bite hard. I discharge every cap with an insulated-handle screwdriver across its terminals before I touch it with my fingers. I do not do live voltage tests with covers off unless I am being very deliberate. The alternator under load is at 120V or 240V — same as house current and just as dangerous. I never backfeed a house through a wall outlet with a generator — that can kill a utility lineman on the grid and is illegal in every state. See generator safety.
When I would call a pro.
If I have run through the four suspects and still have no output, the next stop is the stator or rotor winding — and that means a megohmmeter and a real understanding of three-phase alternator theory. A shop diagnostic runs $75-150. A new stator on a portable runs $80-200; on a 4,000W+ alternator it can hit $400+. At that point on a $500-700 portable, a new generator usually wins. For standby units (Generac Guardian, Cummins, Kohler) at $4,000+, factory-authorized service is worth the call because the controller has diagnostics that point straight to the failed component.
The diagnostic order I actually use
1. Reset the output breaker. (5 seconds, free.)
2. If still dead: the 12V flash to restore residual magnetism. (5 minutes, free.)
3. If still dead: discharge and test the capacitor. (10 minutes, $10-25 to replace.)
4. If still dead: test the AVR. (15 minutes, $20-60 to replace.)
5. If still dead: pull the brushes and inspect (if my unit has them). ($10-20 to replace.)
6. If still dead: stator or rotor winding — shop time, or a new generator.
Most cases I see end at step 2 or 3 with the lights coming on.
IMAGE_NEEDED: Photo of a typical AVR removed from a generator, leads identified, captioned “Typical AVR — small encapsulated board, four wires. I photograph the colors before I unplug anything.”
Video walkthrough
Frequently asked questions
What is the difference between AVR failure and capacitor failure symptoms?
I see dead capacitors give zero output, full stop. A dying AVR more often gives weird output — voltage too low, voltage fluctuating, or surging that flickers lights. My test order is capacitor first because it is cheaper and easier to confirm with a meter.
Will running the generator with no output damage anything?
No — the engine is happy and an alternator with no load is under less stress than a loaded one. I can troubleshoot at length without hurting the unit. I just do not try to bypass safety devices to force output.
Why do these parts fail more on cheap generators than expensive ones?
Two reasons I have seen consistently. First, cheap units use lower-grade electronics — undersized AVRs, marginal capacitors, thinner brushes. Second, cheap units run hotter because of tighter housings and less cooling. Heat is what kills all this stuff. Honda EU-series inverters, by contrast, use beefier electronics and routinely last 15+ years.
Can I run the unit without the AVR or capacitor?
No. Without excitation, the rotor produces no magnetic field that crosses the stator windings, and I get zero output even with the engine spinning. The AVR or cap is what creates the field. Removing them is not a workaround.
How long should an AVR last on a portable?
On a quality unit run gently — 10-15 years. On a cheap unit run at full output in hot weather — 3-7 years. Heat kills them faster than anything else.
My breaker keeps tripping the moment I plug anything in. Same problem?
Different problem. That is usually a short or overload downstream. I disconnect everything, reset the breaker, start the engine, and check that I read 120V at the outlets with no load. If I see voltage with nothing plugged in, the AVR is fine and my issue is the appliance or cord. See generator tripping breaker.
The electrical-end shortlist
- When a generator runs but no power at the outlets, the problem is the electrical end, not the engine. Four suspects total.
- I always check the output breaker first. It trips more than people think and the fix is free.
- Lost residual magnetism is my second-most-common cause. The 12V flash-the-field restores it in 5 minutes, no parts. About a third of these cases.
- Capacitor failure on brushless portables — $10-25, multimeter on µF mode confirms it.
- AVR failure — $20-60, I test input and output voltages against the manual spec.
- Brushes on units that have them — $10-20, replace when they wear past a quarter inch.
If the engine itself is the issue rather than the alternator end, I start with generator won’t start or generator starts then dies. Brand-specific patterns are in the Generac guides. Specific repair walkthroughs by part live in capacitor and AVR. For replacement parts, I match my model number against listings on Amazon.