Generac Error Code 1900: Undervoltage
Code 1900 means the generator ran but its voltage sagged and stayed low. Generac's diagnostic manual defines it as "Prolonged Under-Voltage — undervoltage due to loss of voltage (below 80 % for 10+ seconds)." 1901 is the sudden version — an instantaneous collapse of output voltage (the manual's threshold is generally read as under 15 V for more than two seconds). 1906 is a missing zero-cross signal from the excitation circuit, the sibling of 1902. All three shut the unit down with a red light; after a 1900 or 1901 the controller shows WARMING UP for four minutes. Generac's owner-level checks are all about fuel: fuel selection (NG or LP) correct, fuel jet in the right position, propane above 30 %, adequate fuel pressure. Then clear (OFF → ENTER → AUTO) and test in MANUAL. If it comes back, the next step in the manual is a "Preliminary Output Voltage Test," which is dealer work.
What code 1900 means
An air-cooled Guardian regulates its output at 240 V by controlling current in the rotor field. The controller reads output voltage continuously. When it stays below 80 % of nominal — roughly 192 V on a 240 V unit — for ten seconds, the controller logs 1900 and stops the engine rather than let a brownout reach your furnace board, refrigerator compressor and sump pump.
Two things can make voltage fall that far while the engine is turning. The engine may be starving — a lean mixture, low propane, or the wrong fuel setting will let the engine slow under load, and voltage tracks speed. Or the electrical side has a fault — the excitation winding, brushes, the zero-cross sensing circuit, or the voltage regulation itself. Generac's support article names the same two families: "fuel supply/selection issues" on one hand, "circuitry, firmware, or voltage production" on the other. The owner-level checks all belong to the first family. The second is measured, not guessed.
| Code | Display | Meaning (Generac wording) | Severity | DIY? |
|---|---|---|---|---|
| 1900 | UNDERVOLTAGE | "Prolonged Under-Voltage — undervoltage due to loss of voltage (below 80 % for 10+ seconds). Controller will display WARMING UP for 4 minutes." | Unit will not run | Check, then call |
| 1901 | UNDERVOLTAGE | "Instantaneous — undervoltage due to sudden loss of voltage." Threshold generally read as under 15 V for more than 2 s (the manual's printed figure is garbled in copies we could check). "Controller will display WARMING UP for 4 minutes." | Unit will not run | Check, then call |
| 1902 | UNDERVOLTAGE | Both zero-cross signals missing for more than 1.5 s — see the 1902 page | Unit will not run | Check, then call |
| 1906 | UNDERVOLTAGE | "Single zero cross missing — undervoltage due to faulty excitation winding, zero cross circuit, or circuit in general. Possible cause: field boost hardware failure (one zero cross missing greater than 1.5 sec)." | Unit will not run | Licensed pro / dealer |
| 1903–1905, 1907–1916 | UNDERVOLTAGE | Named as part of the range in Generac's support index; no individual definitions in Generac's air-cooled documents | Unit will not run | Licensed pro / dealer |
The Mobile Link message for every code in the family is "The generator has experienced an undervoltage alarm. Please contact a dealer." Generac's own pages disagree on where the range ends — the article title says 1900–1916, the body says 1900–1906 — so if your display shows a number above 1906 on an air-cooled unit, treat it as an undervoltage of unpublished sub-type and go straight to a dealer.
Reset procedure (as the manual states it)
- Press the red OFF button.
- Press ENTER when the display prompts (some units prompt twice).
- Press the green AUTO button. On Next Generation (Power Zone 200) controllers, hold OFF for three seconds instead.
- Press the blue MANUAL button and let the unit start with no load. Expect the display to read WARMING UP for four minutes after an undervoltage — that's the controller's programmed delay, not a new fault.
- If you're in an outage, follow Generac's restart sequence: unit OFF, main line circuit breaker (MLCB) open, household loads off at the panel, then AUTO; once it runs, MLCB on and loads back one at a time.
Generac's clear-alarm article is explicit: "If an alarm reappears after clearing, contact an authorized Generac service dealer rather than repeatedly clearing."
Step-by-step diagnosis
- Fuel selection. In the controller's menu, confirm the fuel setting matches what's piped in — natural gas or propane. A unit converted to LP by a hurried installer and left on NG runs, but it runs weak and the voltage follows. Do not change the setting yourself; if it's wrong, the jet and possibly the regulator are wrong too, and that's the installer's correction.
- Fuel jet position. Generac's undervoltage article lists "confirm fuel jet position is correct" as an owner step. In practice this is a visual confirmation against the conversion label inside the enclosure, not a job to do with the gas on. If you're not certain, leave it for the technician.
- Propane level. Above 30 %. A low tank cannot deliver vapor fast enough under load, especially in cold weather — the engine leans out, slows, and voltage sags into 1900 territory.
- Fuel pressure. Generac's step reads "check fuel pressure adequacy." An owner can check for the obvious — fuel valve fully open, no one turned off a gas appliance line by mistake, the utility hasn't reported low pressure. The actual measurement (inches of water column at the regulator, under load) needs a manometer and a technician.
- Clear and test in MANUAL. Run unloaded for five minutes after WARMING UP finishes. Steady note, no hunting, no re-alarm → put it in AUTO and let the next exercise cycle confirm.
- Did it trip under a big load? If 1900 posted just as the AC compressor or well pump started, size is in play. Run your loads through the wattage calculator. A 1600 underspeed in the log alongside 1900 makes that certain.
- 1901 (sudden collapse). Output went from normal to almost nothing in a moment. Generac gives no owner-level cause beyond "sudden loss of voltage"; a fuel problem rarely behaves that abruptly, which leaves the electrical side. Clear once, test once; if it repeats, stop.
- 1906 (single zero-cross missing). The controller lost one of its two AC sensing signals. Generac's causes are the excitation winding, the zero-cross circuit and "field boost hardware failure." No owner check exists for this; the manual's action is the Preliminary Output Voltage Test. Dealer.
- Repeats after the fuel checks? Generac's line: "contacting a Generac IASD for diagnosis is strongly recommended." The voltage test is done with the unit running — a multimeter job inside a live enclosure. Leave it to the dealer.
Parts, numbers, and consumables
| Part | Part number | Fits | Where verified |
|---|---|---|---|
| Air filter | 0J8478S | 14–26 kW air-cooled | Owner's manual A0001846499 |
| Air filter | 0E9371AS | 10 kW air-cooled | Owner's manual A0001846499 |
Brushes, the rotor and stator, and the excitation circuit components vary by engine and controller generation. No part numbers are listed here because none could be confirmed with fitment from a Generac or dealer listing; the dealer will quote them after the voltage test tells them which one failed.
Repair or replace?
When 1900 traces to fuel — a low tank, a wrong setting, a marginal gas line — the fix is a service call plus the gas contractor's work, and the generator itself is fine. A brush set is inexpensive. A stator or rotor is not: on an older unit, an alternator repair can be a significant fraction of the cost of a new generator, and that is the point where an honest dealer will lay out both numbers. Ask for the voltage-test readings in writing before deciding.
When to call a dealer
- 1900 returns after the fuel selection, propane level and fuel valve are confirmed.
- 1901 or 1906 appears at all — there is no owner-level check for either.
- You suspect the fuel jet, regulator or fuel selection is wrong — that is gas-train work for the installer.
- The engine runs steady but the house voltage reads low on a plug-in meter — output side, not fuel.
- Generac support: 1-888-436-3722.
Parts and supplies mentioned on this page
A remote propane gauge removes the most common cause of a winter 1900 before it happens; a plug-in voltage meter in the house tells you at a glance whether the generator is holding 120 V under load.
- Propane tank gauge / remote LP level monitorCheck price on Amazon
- Digital multimeter (AC/DC, auto-ranging)Check price on Amazon
Related codes and symptoms
1902 is the most-searched undervoltage code and has its own page. 1600 Underspeed is the engine-side twin — if the log shows both, the engine slowed first and voltage followed. 2100 Overload is the current-based cousin. If the unit runs, posts no alarm, and the house is still dark, the problem is the transfer switch, not voltage — see runs but no power.
Not sure the generator can carry the load?Generator Wattage Calculator Running watts, the largest starting surge, and a generator class from manufacturer charts.Frequently asked questions
What is the difference between Generac 1900 and 1902?
1900 is prolonged low voltage — output stayed below 80 % of nominal for ten seconds or more, usually a fuel-supply or load problem. 1902 means both zero-cross sensing signals disappeared for more than 1.5 seconds, which points at the excitation winding or sensing circuit rather than fuel.
Can I reset a 1900 code myself?
Yes — OFF, ENTER, AUTO, then a test in MANUAL. Generac lists the fuel checks first and says to call a dealer if the alarm reappears rather than clearing it again.
Why does the display say WARMING UP after an undervoltage?
The diagnostic manual notes that after 1900 or 1901 the controller displays WARMING UP for four minutes. It's a programmed delay before the unit will accept load again, not a separate fault.
Can low propane cause code 1900?
Yes. Generac's article asks for the LP tank above 30 %. A low tank on a cold day can't supply enough vapor under load; the engine leans out and the output voltage drops with it.
What is Generac code 1901?
An instantaneous undervoltage — a sudden collapse of output rather than a slow sag. Generac gives no owner-level cause for it; clear once and test, and if it repeats, call a dealer.