Generator keeps cutting out after about 20 minutes - faulty AVR or something else?

Had exactly this with my Honda EU22i about two years ago — classic symptom pattern. The 20-minute window is the giveaway; that's roughly how long it takes for the engine to reach full thermal equilibrium.

Worth working through these in order before condemning the AVR:

1. Fuel delivery
Carburettor needle jet partially blocked. Heat causes fuel to vaporise unevenly once the carb body gets hot. Clean it properly with an ultrasonic cleaner if you can.

2. Low oil shutdown
If the oil level is even slightly marginal, the thermal expansion of everything internal can trigger the low-oil sensor prematurely. Top it up and test.

3. Capacitor on the AVR itself
On Chinese inverter-generators (Kipor, Hyundai, etc.) the capacitor on the AVR board degrades with age and heat. Costs pennies to replace but you need to match the µF rating exactly.

4. Cooling/airflow
Are you running it in an enclosure or somewhere restricted? Generators need proper airflow — surprising how many people box them in thinking they're being clever about noise.

What generator model are you actually running? That'll narrow it down considerably. Also — is it cutting out completely (engine stops) or just losing load (engine keeps running but output drops)? Those are two very different fault trees.

If the engine is stopping, I'd bet on thermal protection or fuel starvation before touching the AVR. If output drops but the engine runs on, then the AVR or capacitor becomes more likely.

Happy to help diagnose further if you can share more details.

@Bazza60 interesting that the thermal threshold lines up so precisely — has anyone ruled out the low oil shutdown sensor giving a false trigger? Mine did something similar in my van build; turned out the sensor itself was faulty rather than actual low oil. Worth checking the actual oil level first obviously, then if that's fine, disconnecting the sensor temporarily to see if it runs past the 20-minute mark. If it does, you've found your culprit. Replacement sensors are fairly cheap. Also — is the generator on a level surface? Those Honda sensors can be surprisingly sensitive to even a slight incline when oil shifts around under heat.

@PaddyGibson the low oil sensor going flaky is basically a rite of passage with these — mine did identical on the narrowboat and I nearly binned a perfectly good genny over it.

Good shout from @EssexNomad on the oil sensor — worth checking, but before pulling anything apart I'd also look at the air filter and cooling fins. If they're clogged with dust or debris, heat builds up faster than it should and the thermal cutout does its job a bit too enthusiastically. Had a friend with a similar unit where a partially blocked air filter was causing exactly this 20-minute shutdown pattern. Give the fins a good blow-out with compressed air and see if that shifts the threshold at all. If the generator then runs longer before cutting, you've likely found your culprit. Simple and free to check before going down the AVR or sensor replacement route. What's the maintenance history like on it? Sometimes these things just need a thorough clean after a season of hard use.

@WillStevens good timing on that — curious what the airflow angle is you were going to mention?

I'd add one thing nobody's flagged yet: fuel starvation. Specifically the fuel cap vent. Had almost identical on my backup genny last winter — 20-odd minutes then cuts. Turns out the cap vent was partially blocked, creating vacuum in the tank as fuel drew down. Cracking the cap slightly and letting it run cured it immediately.

Dead cheap thing to rule out before you're elbow-deep in sensors or AVR testing.

• Loosen cap slightly, run it
• If it stays up, you've found your culprit
• New vented cap is pennies

Doesn't rule out the oil sensor obviously — @PaddyGibson is right that flaky sensors are proper common on these — but start simple before you go diagnostic.