The generator started. The transfer switch did not.

You left the generator running to keep the batteries up and the fridge cold, came back after four hours, and found a warm fridge and a bank lower than when you left. The set ran the whole time. You can hear that it ran — it is still warm, and the tank went down. It simply was not feeding the coach.

Here is the honest position before anything else: we cannot read your generator. There is no generator in our catalog, no transfer switch, no auto-start controller and no run-hour meter — not supported, not in progress, not anywhere. Nothing broadcasts generator state, so there is no reading to alert on.

What there is, is electricity. A generator that is doing its job changes readings you already have, and one pairing catches the failure at the top of this page — the set running while nothing reaches the coach — which is the failure worth catching, because it is silent, it wastes fuel, and every consequence of it shows up hours later somewhere else.

What the electricity lets you infer

a. The current changing sign

The best generator-start signal in the catalog needs no AC-side device at all. A SmartShunt reads current with a sign, so a bank that was discharging and is suddenly charging means something has handed the converter a live source. If shore power is dark at the same moment, that something is the generator.

The flip back to negative is the set stopping, or running out of fuel, or the transfer switch dropping out. It reads every 60 seconds and needs its Instant Readout key pasted in first, and the one thing that ruins it is daylight: solar does exactly the same thing to the sign, and daytime is when people run generators. On a rig with panels, this method is a night-time method.

b. Line frequency, which only works one way round

A Power Watchdog reports line frequency, and the grid holds 60 Hz far more tightly than a laboring engine does. So frequency wandering outside about 59.5 to 60.5 is real evidence that you are on engine-driven generation rather than the grid, and a good sign the set is loaded harder than it likes.

c. Where your Watchdog is decides what it can see

This one is about your install rather than the device. A portable unit lives on the end of the cord at the pedestal, so while you are running on the generator it is measuring a post nobody is connected to — it sees nothing, and its silence means nothing. A unit bolted into the electrical bay sits downstream of the transfer switch and reads whatever is actually feeding the coach, generator power included.

Which is worth knowing before you try to read any of this: on a portable unit, most of the AC-side methods on this page are simply unavailable while the generator is the source.

d. Heat and fuel, the only proof the engine ran

Every electrical method above tells you the coach was fed. None of them can tell you the engine ran, and that difference is the article. A wired probe in the generator bay — a Govee H5044 at 30 seconds, an Inkbird IBS-P01B at 120 — shows heat that nothing else explains. On an LP set, propane stepping down on a Mopeka or a SeeLevel’s propane channel is the same evidence from the fuel side.

Both are slow and neither is precise. Bay heat lags the start and lingers after the stop; propane at 300-second polls moves over an hour, not a minute. They are not there to tell you the moment it started. They are there to say the engine ran at all, which is the half no ammeter can supply.

The pair that catches the transfer switch

Put those two halves together and you have the failure this article is named for. The engine ran — bay heat, or fuel going down — and the DC side never started charging, with shore power still dark. The set started and the coach was never connected to it.

Neither half says it alone. Heat or fuel burn only proves the engine turned; the flat DC side only proves nothing arrived. Together they are unambiguous, and they are the reason to put a probe in the generator bay even though a probe in a generator bay sounds like a strange thing to own.

The combinations

More than anywhere else in the cookbook, this family is nothing but pairs: no single reading here says the word generator.

• Shore dark, and the shunt current flips from negative to positive — the generator started and the transfer switch did transfer. This is the good case, and it is the one you will see most.
• Bay warming or fuel dropping, and the DC side flat — it started and nothing reached the coach. Transfer switch, its breaker, or the set’s own output breaker.
• Watchdog live, frequency wandering out of band — you are on engine power and the set is working hard. Shed some load; this is the same reading a sagging pedestal uses, read from the other direction.
• Watchdog live and normal, EasyStart reporting current — the air conditioner is running on whatever the source is, which on a dark pedestal means the generator is carrying it.
• Hub goes offline, and the shunt’s last reading flipped negative in the same minute — the set stopped and your hub was living on generator power. Nothing further is coming until something else feeds it.
• Watchdog live at normal voltage with a non-zero error code — not a generator event at all. The EMS is refusing the source, whatever the source is, and on a generator that is worth taking seriously rather than bypassing.
• An EasyStart the hub cannot reach — nothing. It only answers while the air conditioner has power, so its silence is normal and diagnoses neither the generator nor anything else. Listed because it is the pair everybody builds first.

What none of this tells you

Run hours. The readings that count things — lifetime starts, energy used, fault counts — only ever go up, and rules fire on a reading crossing a fixed line, so there is no “tell me when it has run for three hours” to set. That is a chart you read, not an alert you arm.

The changeover itself, usually. A transfer switch takes a few seconds including its own delay, and the Watchdog polls every 30, so the event generally happens between two readings and is simply not in the record. You see the state before and the state after. And the EasyStart broadcasts a status code we show as a number: there is no table behind it in the app, so treat it as something to compare against itself rather than something with meanings.

What to do about it

If you already own a shunt, you have the useful half for nothing: watch the sign, at night, and you will see the set start and stop. That alone catches most of what people want here.

Add a probe in the generator bay only if you run the set unattended — to keep a fridge cold while you are out, or to top a bank up on a schedule. That is the case where the transfer-switch failure is expensive, because it is silent and you find out four hours later, and it is the only case where a temperature probe in a generator bay is a reasonable thing to buy.

And do not build much on frequency. It is the one reading here that looks like a clean generator detector and is only half of one.

Where to get one

Nothing on this list is bought for the generator. The shunt is a battery instrument, the Watchdog is shore power protection, the probe is a fridge thermometer — they are what you already have, read differently. The compatibility database has what each one broadcasts.